The Chile international has travelled to Scandinavia ahead of friendly matches against Sweden and Denmark, but he almost dropped out of the squad after a disappointing spell with the Red Devils.
Per Charlotte Duncker of the Manchester Evening News, Sanchez said:
"As I am self-demanding, I expected something better. After my arrival at United, it was hard to change everything very quickly. I even hesitated to come here [to Sweden, with the national team].
"I had asked permission to miss these games, but then I thought better and spoke with Claudio [Bravo] and told him that we should all be united.
"The change of club was something that was very abrupt—it was the first time I've changed clubs in January—but many things have happened in my life that are difficult."
Sanchez arrived from Arsenal on January 22 but has scored just one goal and laid on two assists in 10 matches for the Red Devils.
Following United's UEFA Champions League exit at the hands of Sevilla in the round of 16, the 29-year-old has failed to improve on a poor record in Europe's elite competition since moving to the Premier League in 2014, per Opta's Duncan Alexander:
United legend Rio Ferdinand discussed Sanchez's struggles on BT Sport:
His form with Arsenal was well below his usual standard. He hit 30 goals and assisted 19 for the Gunners in all competitions last season and left the club with just eight and four in the 2017/18 campaign, respectively. So it is perhaps unsurprising he has not managed to immediately turn it on despite the change of scenery.
He also joined the Red Devils at a difficult time in the season, with the team's performances frequently lacking over the last two months.
Fatigue could be playing a role in his poor form as well.
With Chile, Sanchez has taken part in the 2014 World Cup, the 2015 and 2016 Copa America finals and the FIFA Confederations Cup last year. The team reached the final in each of the latter three, so he has not had a summer off to recuperate for some time.
From that point of view, it could be a blessing of sorts that La Roja missed out on the World Cup in Russia this year, since Sanchez can get some much-needed rest.
Football writer Liam Canning believes he may be able to get back to his best next season:
Liam Canning @LiamPaulCanning
It’s clear that Alexis Sánchez isn’t performing to the level he was at Arsenal. I don’t think we’ll see the best from him until next year when he has a good pre-season in him. Mourinho has to stop unbalancing the attack just to get him in the team, though. Rashford/Martial LW.
Now that United are out of the Champions League and look to have second place in the Premier League wrapped up—they are two points clear of Liverpool with a game in hand—it's not a disaster if the team do need to wait until after the summer to see him at his best.
The Red Devils are still in the FA Cup, though, and it's always a positive to end the season on a high. United will hope he can improve in the coming months even if he doesn't hit the heights he's capable of.