The Ecuadorian winger was one of the Red Devils' most consistent players last season, and arguably their top performer in the final few months.
But with Wilfried Zaha due to arrive in the summer, Valencia's poor form could see his position at Old Trafford further marginalised.
He is no longer a regular starter, and has been kept well clear of United's most important games in 2013.
Why is this? What is behind Valencia's rather sudden loss of form?
Here are five possible reasons.
In November last year, it was reported that Antonio Valencia had picked up a minor hip injury and could be out of action for a week (via ManUtd.com).
Sir Alex Ferguson said at the time: "He's had a bit of pain there for the last couple of weeks but he's been playing with it. That's the type of guy he is. He never mentioned it and plays on."
Having been a regular starter through the months leading up to the knock, Valencia was subsequently seen less and less.
He made only two appearances in January—both from the bench—while starting just a couple of games in February.
His horrific broken leg suffered in 2010 could also be rearing its ugly head again.
In an interview with The Mirror, the man himself revealed: "The injuries I've had have held me back a bit. I still get the odd twinge from the last injury I had to my leg and that perhaps affected my progress slightly" (h/t Sky Sports).
These problems don't seem to want to go away and could be a key reason for his drop in from.
Antonio Valencia's distinct lack of a left foot has been one of the worst kept secrets in football.
The winger can only move and cross with his right—you can be sure every opposing full-back in the country has been well-drilled on the fact before facing him in action.
In the past, Valencia has used his timing and explosive burst of pace to beat a defender, but with both having vanished this season, he has been left redundant as an attacking threat.
Without that extra yard of space, he has been able to deliver far fewer balls into the penalty area for the likes of Wayne Rooney and Robin van Persie to attack.
He has been left stranded on the wing, unable to cut inside or streak down the right flank as he's usually accustomed to doing.
This past summer, Antonio Valencia was handed the No. 7 shirt vacated by the departing Michael Owen.
The No. 7 shirt that has been worn by the likes of George Best, Bryan Robson, Eric Cantona and David Beckham in the past.
The shirt and the number carry such a rich and meaningful significance for the club, the pressure placed on its current recipient has the potential to be extraordinary.
As noted before, Valencia ended last season in top form and was expected to carry such impressive displays into this term.
The hefty pressures that come with being a Manchester United player can often weigh heavily on a man's shoulders.
The No. 7 shirt only adds to that weight.
An issue similar to the previous slide, Antonio Valencia's problems this season seem to be as mental as they are physical.
He has lacked the confidence that wingers so desperately need in order to perform—the confidence to take on a defender and to deliver a cross at the first time of asking.
When he has taken to the field his body language has seemed reluctant—more afraid of failing than eager to succeed.
It is difficult to pinpoint quite where this has come from. Perhaps something has happened to the man which has yet to be reported.
A month or two in the shadows can be forgiven, but for a player to go through a whole season lacking any real mettle is very worrying indeed.
Before this season, Antonio Valencia was known as one of the elite crossers in the Premier League.
Yes, he has been unable to get into good enough positions to deliver quality balls into the box but even on the odd occasion when he has, his final product has just not been good enough.
Rafael has looked by far the more penetrative threat down the right flank, the Brazilian full-back being one of the Red Devils most improved players.
Valencia has only a single assist and no goals to his name in his past 18 appearances for United in all competitions.
It is a testament to how poor he has been that Danny Welbeck is considered a better option on the right wing right now than him.
Why do you think Valencia has been so poor this season?