The Sky Sports pundit is backing the Hammers in no uncertain terms.
Although he praised them for sitting in the current top four, Merson believes Manchester United are not getting the criticism they deserve simply because Van Gaal is a good talker.
West Ham, meanwhile, have a strong chance of victory when United travel to Upton Park on Sunday afternoon - according to the pundit.
"It's a hard place to go. Even though the Hammers were poor at Liverpool last week, I think they have got a really good chance of winning this game," Merson said.
"Van Gaal is a good talker, that's why he's not getting a hard time, and to be fair they're in the top four. That's what they were looking for this season, a bit of stability - but this game is not a foregone conclusion."
West Ham are currently eighth in the top flight, a disappointing return considering they have spent so much of the league campaign so far in and around the Champions League places.
Manchester United, by contrast, sit third in the standings, although there is only a seven-point gap between the two sides. And, if Merson's prediction bears fruit, that gap will be reduced to just four points.
After the Manchester United clash, West Ham face three away games in a row in the Premier League, traveling to Southampton, West Brom and Tottenham Hotspur.