The defender, who moved to St Mary's Stadium from Lyon last summer, has been one of the star performers for the Saints this season, but will miss the next eight weeks of action, including the visit of the league leaders tonight, after suffering ankle ligament damage earlier this month.
"Southampton seem to have hit a bit of a brick wall after their brilliant start to the season," Merson told Sky Sports News. "They've got a few injuries in key areas and maybe that's because of the way they play.
"They play a closing-down game and that's tiring when you've got to do it all of the time. Dejan Lovren is out for a little while and that is a massive miss for them, so I expect Arsenal to carry on their good form.
"Their away record is phenomenal and I'm going for another win here."
Southampton against Arsenal gets underway at 7.45pm tonight.