Not just because of the huge amount of money the Chile international was to pocket as part of his switch from Arsenal, but also because Mourinho has always held reservations about integrating an elite player midway through a season.
Nine months on from Sanchez's transfer, it would be fair to conclude it has not yet been a success—but Saturday's winning goal against Newcastle United may yet prove to be a catalyst for better days.
Visa issues and a lack of team harmony are just two areas of frustration that have held him back since the move to Manchester. But now may come a time when United's boss puts faith in him—and the 29-year-old proves he is still one of the most exciting attacking talents in the game.
His Premier League bow for United came on January 31 at Wembley Stadium against Tottenham Hotspur, but it was an evening he would prefer to forget: Firstly, because United lost 2-0 and also because he had nowhere near the impact that was expected.
Spurs squeezed him out of the game, surrounding him when in possession and limiting the times he could receive the ball in dangerous areas. He had 75 touches, according to Sky Sports, the second most of any United player in that fixture. But he failed to create a single goalscoring opportunity or attempt a shot on goal.
He started the match on the left flank and later switched to a more central role behind Romelu Lukaku. But the way he struggled to fit in that night has become a common theme for the 23 matches he has featured in since. He has scored just four goals and assisted six times, but on the whole he just has not looked comfortable in Mourinho's system.
The output is not what you would expect of a man who earns the most money at the club and one source, an agent based in Manchester who did not want to be named, told Bleacher Report why he feels Sanchez may have struggled to recover from a slow start to life at Old Trafford.
"We must remember that Alexis chose United because of the money," the insider explains. "He wanted to join Manchester City, but they would not pay the same salary United were willing to stump up. And as a result his whole outlook changed. You have to wonder how he looks back on that moment now. He turned down the chance to become a Premier League champion under Pep Guardiola and instead he has been struggling along at United, who have been in chaos."
Sanchez also leaned towards United because of their history. In Chile, many people support the club. It excited fans there far more than a move to City would have.
Other sources in Manchester also suggested there is a feeling Sanchez regrets his decision to wear red in the city rather than blue. His body language and performances have rarely shown him to be any happier than he was in those final miserable days at Arsenal.
Sanchez could not wait to pack his bags and leg it out of the Emirates Stadium. He forced a situation whereby the club knew he would not be committing himself to a new contract and took the chance to cash in while they could.
Then-Gunners manager Arsene Wenger did not like the idea of him moving to United, but he was powerless to stop it. Many figures in the dressing room were less bothered about where he landed, they were just pleased to see the back of him.
In a feature called Alexis Sanchez's Lonely Last Days at Arsenal, I uncovered Sanchez had been unhappy for months before his move to Manchester materialised, and how teammates began to see him as a brat.
Additionally, journalist Raphael Honigstein told The Totally Football Show podcast (h/t the Mirror), quite how bad the atmosphere became around the Chilean.
"I think it's been under-reported just how badly the whole Arsenal dressing room hated Alexis Sanchez," he said. "They really felt that he was just playing up to the camera doing all these things like 'Oh I'm trying but nobody else is! What am I doing here?'
"Arsenal would put up stats inside the dressing room last year after every game showing the amount of kilometres run, the passing, and Sanchez would regularly come out with the lowest mileage by a big distance. The players would just go absolutely bananas at him playing to the camera and making all these moves like 'Oh, they're just not worthy of me! What am I doing here?' So they really wanted to get rid of him."
Sanchez was given an extremely healthy pay package to convince him to turn his back on a dream to join City, which he had held for the best part of a year, and instead sign on at United. Yet it has not been enough to satisfy him. Insiders at Old Trafford told B/R how they felt he has not seemed at ease living in the north.
Speaking to Sky Sports' Premier League Preview show last weekend, experienced journalist and Arsenal expert Matt Scott questioned whether United should have bothered with the signing:
"He had a great reputation for goals and assists when he was at Arsenal, but that was because of the chemistry he had with Mesut Ozil, which was unique. He's in a far more rigid system than he was used to at Arsenal, and frankly he was the wrong player for them to purchase—particularly at £26 million per year. He's been out of position and out of place altogether at the club."
Sanchez has found himself as a substitute for United recently, but it could be that the view from the bench has given him another chance to figure out how he can be most fruitful in this team.
One of the main reasons Mourinho does not like signing players in January is because they do not have time to understand his methods and philosophy. Sanchez has not found it easy to get to grips with the setup and was not helped when he had further issues in the summer. He could not join up immediately with the team on their United States tour because of problems with his visa that the club did not address efficiently.
The former Barcelona man's main role has been on the left flank, but those close to him believe that could change in the next batch of fixtures after the international break.
Romelu Lukaku has played almost every minute this season, but insiders told B/R they expect Sanchez to be given a central role on some occasions going forward, in a bid to give him fresh opportunity while affording the Belgium international a brief break.
"His best position, for me, is still playing off the left as he has done for the majority of his career," explains Chilean sports journalist Matias Grez. "While he did adapt surprisingly well to being the focal point at centre-forward for Arsenal—creating rather than scoring goals has always been his strength—a permanent spot on the left at United with Lukaku up front could really bring out the best in him again."
Sanchez should be feeling fresher than he has done in recent seasons. He had no FIFA World Cup duty over the summer as Chile did not qualify, so he was able to instead spend extra time preparing for club football.
Two months into the season, though, there is no sign of him attaining the form he has shown previously. Will he ever get back to the levels we once saw?
"I think between 2013 to 2016 he was one of the best attacking players in the world—top 10," says Adam Brandon, South American expert for World Football Index podcast.
"His numbers back this up in terms of goal contributions, and he has won trophies at Barcelona and Arsenal, but I think he's probably disappointed he hasn't won more in his career at club level—especially given his team-mate for the national team, Arturo Vidal, has won seven league titles in a row. I imagine Alexis is probably a little envious of that.
"At Barcelona, he would have won more trophies if he had stayed but probably would have been unhappy with the minutes he was getting; at Arsenal, he was a guaranteed starter but under Wenger they rarely had a chance to realistically win the title.
"This year at Manchester United has obviously seen a decline in his performances. We'll have to wait and see if a new manager can change his fortunes around. It has surprised me a bit...although I was apprehensive of him working with a defensive coach like Mourinho.
"I was one of many who expected him to be excellent this season after he finally had a summer off, having been with Chile at a tournament every summer from 2014 to 2017. He's had some off-the-field issues in the past couple of years as well and perhaps just the last few years of being at the top of his game for club and country has taken its toll physically and mentally."
Sanchez is still adored in Chile, where he is regarded as one of the greatest players of all time, but he is a long way from reaching such status with fans in the Premier League right now.
"There's a sense of frustration he's been used badly and Mourinho isn't getting the best out of him, but a firm belief he will at some point rediscover his best form," Grez said of the view of Sanchez back home.
It could go a long way to defining their season, and Sanchez's contributions in those games will be telling.
Soon we will have a better idea of whether United were right to throw cash in his direction—or if this is a gamble that will never pay off.