The former Paris Saint-Germain striker had a fine debut year last season as he netted 28 goals in 46 appearances in all competitions with United going on to win the Capital One Cup as well as the Europa League.
A serious knee injury suffered in a Europa League quarter-final clash against Anderlecht halted his campaign though, with the Red Devils deciding to release him following the expiry of his contract.
However, Ibrahimovic recovered at a startling pace amid rumours that he would eventually return and is now officially back at Old Trafford, giving United even more squad depth.
But despite being the club's top scorer last season, former Arsenal player Merson believes the move does not make any sense and that it could hinder teammate Henrikh Mkhitarayan.
"I just don't get it, I really don't," Merson said on Sky Sports, as quoted on The Express. "I know he got all those goals last season but who else had a great season last year? (Ander) Herrera, (Antonio) Valencia, no one else."
"Look at Mkhitaryan now, that won't happen if Zlatan Ibrahimovic is up front. Look at the way they were breaking then, it was too slow.
"For me Henrikh Mkhitaryan is one of the best players in the league, when he's got pace around him like that."
With the big-money signing of Romelu Lukaku, it is unlikely that Ibrahimovic will start games up front, however, Merson adds that if the Belgian fails to score, the fans will call for the 35-year-old to play which will affect the team due to his style.
"I know people are coming to me and going 'oh but he won't play'. He's not coming there to sit on the bench," he added. "If (Romelu) Lukaku doesn't score for two games, the fans will be singing his name, he'll come on, he will nick a goal because that's the way he plays and he's still a good player but won't be good enough for Man United, and then he's in the team.
"Then it all slows down again. He's the other side of 35, he's just had a bad injury. I know he keeps himself fit but I just don't get it, I really don't.
"If I was Man City or I was Liverpool or I was Chelsea or Tottenham, I'd be wanting him coming back. I don't think it helps Manchester United."