Arsenal head into the clash with their hopes of a top-four finish all but over following Sunday’s 2-0 defeat to Tottenham.
Meanwhile Manchester United’s best hope of gaining a Champions League place next year now appears to be through the Europa League.
And Merson believes Arsenal will beat United 3-0 as Jose Mourinho will be resting players for the Europa League.
“Arsenal are certainties in my opinion,” Merson told Sky Sports.
“It’s all about timing at the end of the season and this game is perfect timing for the Gunners. United could win their next four Premier League games 5-0 and still finish fifth in the league.
“On the other hand, they might only have to win two Europa League games to qualify for the Champions League. I don’t see anything but wholesale changes for Jose Mourinho for this game.
“It would be interesting to see what would happen with Mourinho’s selections if Manchester City lost to Crystal Palace.
“It would present United with another opportunity of sneaking into the top four and while he has said he will rest players, would a negative result for City change his mind?
“I don’t think so. I’d be shocked if any of the big guns played at the Emirates on Sunday.
“The wholesale changes put severe pressure on Arsene Wenger. They will be playing a second string side in my opinion and if they can’t beat an under-strength Manchester United, huge pressure will be put on the Frenchman. He would probably prefer to be playing United’s first team.
“I’m backing Arsenal for a big 3-0 win and before Manchester United fans go ballistic I’m only backing that result because of my theory. If it was different I’d back the draw because Mourinho can get a draw at will.”
