The Red Devils are currently in fifth place in the Premier League table, and they are looking to catch Manchester City in fourth.
United have put pressure on their city rivals recently, but there is an added complication due to the fact that West Ham United are also in the mix.
The Hammers are only one point behind United, and Louis van Gaal’s side still have to visit the Boleyn Ground before the end of the season.
Holloway is hoping that West Ham can come out on top, but he feels that United look the most likely side to claim the final Champions League qualification spot.
"I always look at Manchester United," the former QPR boss said, when asked who would make the top four. "I know this season they’re having. Losing Sir Alex, I don’t think they’ll ever be the same again.
"But I’m always looking over my shoulder for them. I’d like West Ham to do it, if I’m being honest, but you cannot write off Manchester United. They look quite ominous."
City currently hold a four point cushion over Manchester United and they are five points clear of West Ham, but the challengers both have a game in hand.
City’s final two matches come against Arsenal and Swansea City.
United’s remaining fixtures are against Norwich City, West Ham and and Bournemouth.
West Ham have games against Swansea, United and Stoke City left to play.