Smalling, who won the Players Player award last year, revealed he put Valencia ahead of Zlatan Ibrahimovic, who has scored 28 goals for United this season, because he was the most consistent performer in the team.
"I think Antonio has been so consistent, at times he's been our outlet, whether he's been playing at right-back or on the right wing," Smalling told MUTV.
"He's been so dependable as well, not just in an attacking sense, but also when he's been defending.
"Any winger or striker that comes up against him has to get around him. That means they have to be stronger or quicker than him, and neither of those is easy!"
"Antonio would be a worthy winner in my eyes."
United will have their annual awards ceremony on Thursday night at Old Trafford.