Ibrahimovic rejoined Manchester United yesterday, putting pen to paper on a one-year deal and taking a £200,000 pay cut in the process.
United have set the early pace, and are top of the Premier League table, following their 4-0 wins against West Ham and Swansea respectively.
Romelu Lukaku, who joined in a £75million deal, has hit the ground running, scoring in both games.
Ibrahimovic, who missed the end of last season with a serious knee injury, is expected to return to action in the second half of the campaign.
And Merson is worried about the effect it could have on Lukaku.
“Lukaku is top man now, you can see it on the pitch. That’s going to change!,” Merson said on Sky Sports.
“He’s got a big presence, Ibrahimovic, and he hasn’t gone to United to sit on the bench.
“If Lukaku goes a couple games without scoring, the fans will be cheering Zlatan’s name, and Lukaku could go into his shell, and we saw that last season in big football matches for Everton, he went into his shell.
“I don’t think he’s gone there to fizzle out and finish his career. When you watch them at the moment, United are a completely different side to last season. They break with real pace.
“Last season Zlatan was good, but nobody else was. That doesn’t win you the Premier League, everybody has to be good. I just think he slowed the game down last season.”
United take on Leicester tomorrow (5:30pm).