The Frenchman was scrutinised for a largely indifferent performance in Tuesday's 2-0 Champions League defeat by Barcelona, extending his goalless streak to eight games in the process.
In contrast to Arsenal's European disappointment, the Red Devils thrashed Midtjylland 5-1 on Thursday night to move into the last 16 of the Europa League, but Merson says Arsene Wenger's side will bring them back to earth this weekend.
"Both sides have had European commitments in midweek but I'm not looking too much into that," the pundit told Sky Sports. "I'm not sure their midweek exertions will have much of an effect and I see Arsenal coming out on top.
"They are the favourites with the bookmakers and I can understand why when you look at the recent form of both teams. Olivier Giroud is the man I'm backing to score first with Arsene Wenger's side enhancing their title claims with a 2-1 win at Old Trafford."
Victory would be the perfect way to end a difficult last seven days for Arsenal.
Not only did Barca leave their Champions League quarter-final hopes hanging by a thread, but Hull City earned a hard-fought 0-0 draw at the Emirates last Sunday to force a fifth-round FA Cup replay.
They sit third in the table, behind Tottenham Hotspur on goal difference and two points adrift of Leicester City.