And Mersonalso reckons executive vice-chairman Ed Woodward could face the axe, with the pressure on the two Red Devils bosses after a miserable campaign.
United got Ighalo on loan from Shanghai Shenhua as a replacement for the injured Marcus Rashford.
Norwegian striker Erling Haaland, now at Borussia Dortmund and Bournemouth’s Josh King were among a variety of late options before United settled for Ighalo.
Ighalo, 30, scored 16 times in 55 Premier League appearances for Watford but Merson doubts whether he has the ability to shine at Old Trafford.
Writing in his column for the Daily Star, Merson, a league, FA Cup, League Cup and Cup Winners’ Cup winner at Arsenal said: “Odion Ighalo could be the final straw at Manchester United.
“If that signing is a disaster then Ed Woodward and Ole Gunnar Solskjaer are in big trouble.
“Don’t get me wrong. He could rediscover the form he once he had at Watford and be a huge success. But he could be the biggest laughing stock the Premier League has ever seen.
“Only time will tell.
“But remember, the last time he was here he only scored once in 18 games before Watford sold him and he’ll have gotten lazy playing in China.”
Merson pointed to comparisons with Celtic and Sweden legend Henrik Larsson, who joined United on loan aged 35 and impressed in his short stint in 2007.
But the France 1998 World Cup star does not believe Ighalo has the experience to emulate the Swede’s success.
He added: “I’ve heard people compare it with Henrik Larsson because nobody expected him to still be any good when United signed him either.
“But Larsson had been world class. He had scored goals in major tournaments and won big trophies. And he still had his finishing and his movement.
“Ighalo doesn’t have any of that on his CV and he’s coming into a struggling team. The biggest one he’s ever played for.
“It’s a big ask, and if it all goes horribly wrong it will just pile more pressure on the manager because you’re always judged on your signings as well as your results.”
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