Ecuador international Valencia, 26, was the difference in his side's emphatic 5-0 win at Wolverhampton Wanderers, as he scored, made two assists, created a staggering seven chances for teammates and won the Man of the Match Award.
It was a result that extended United's Premier League lead to four points, temporarily at least, with bitter rivals City not in action until Wednesday when they take on Chelsea at Eastlands.
Watching Valencia's display, it would appear he's never been away from the side, despite the fact the game at Molineux was his first match in over a month, following the hamstring injury he sustained in United's 2-0 UEFA Europa League win at Ajax.
Following the match against Wolves, Sir Alex said of his best winger:
He could be very important [in the title race]. His form before he got the injury was outstanding. Now he is back we hope he stays back fit. He is such an honest, hardworking player.
You get two sides to Antonio. He is prepared to work really hard. He can tackle, he can run, but he can also beat a man. He has got everything really.
Given how United focus so much of their forward play on the wings, clearly having in-form wingers will be vital in the Premier League title run-in.
At the moment, and throughout this season, Valencia has been the team's best performing winger.
This term he's scored three goals in the league and made 11 assists—the second-most amount of assists behind City's David Silva (12), and ahead of players such as Juan Mata, Gareth Bale, Theo Walcott, Nani, Samir Nasri and others, despite having much less playing time this season.
His ability to hit those high-quality key passes—which turn into assists when the player on the receiving end of the pass scores—has been paramount for United throughout the campaign, and will be in the title run-in, seen as City have a better goal difference.
In some relatively easy matches ahead of that crucial, highly-anticipated derby clash at the Etihad Stadium on 30 April, the Red Devils will need to keep up with their blue counterparts in the goalscoring department, especially if the title race does boil down to goals scored.
And if there's one thing Valencia provides for the team better than any other midfield option United have, including Ashley Young and Nani, it's goals.
The 11 goals Valencia has set up for teammates this season have come from just 41 key passes—meaning that one in every four chances he sets up for a colleague will be scored.
That key pass to assists ratio is the best in the Premier League, and given how consistent United's top South American has been this campaign, his ratio doesn't look like slowing down in the season's business end.
And besides creating goals, Valencia, as Sir Alex said, is also very useful in defence, being United's best defensive option in midfield.
It's no coincidence United concede less goals when Valencia plays, as he's statistically the second best winger defensively in the Premier League, behind Chelsea's Ramires.
So far this season the Ecuadorian has won 46 tackles at a rate of 2.6 per game (second only in the United team to Michael Carrick and Rafael da Silva), and made 22 interceptions at a rate of 1.2 per match.
In comparison on the wings, Nani—who has made six more league appearances and eight more starts—has won just 31 tackles, at a rate of 1.3 per match, while Ashley Young has won 1.1 tackles per game.
As Sir Alex says, Valencia's ability defensively is just as important as what he can do going forward.
Considering United play a lot of their football on the wings, it's perhaps most crucial in their remaining nine games to have a winger who will consistently supply high-quality goalscoring chances—which Valencia has proven he provides.
And seen as a lot of their games revolves around the flanks, drawing the full-backs forward, it's also fundamental to have a winger who not only tracks back well, but can get stuck in and win back possession, so the team are not left vulnerable by the opposition exploiting pockets of space out wide.
Again, Valencia provides that for his team.
Possibly the best old-school winger in football, Antonio Valencia is the key man in the Premier League title race itself.
Because when he's on form, Manchester United will be on form. And when they're on form, they'll win matches and keep the distance between themselves and Manchester City.
So no pressure Antonio, but given the way the team's results have seemingly hinged on your performances, it's your form which could well decide which half of Manchester wins an historic Premier League title this May.