Cristiano Ronaldo ruined Ole Gunnar Solskjaer's Manchester United strategy, according to former Premier League star Paul Merson.
The Portuguese re-joined the Red Devils from Juventus on deadline day and Merson believes the Norwegian wasn't prepared for his return.
Solskjaer was sacked by United on Sunday after winning just one of his final seven Premier League games in charge.
During that time, his team suffered embarrassing defeats to Liverpool, Manchester City and Watford - slipping to eighth in the table.
United finished second in the Premier League last season, winning 21 of their 38 fixtures, but they will be lucky to finish fourth this time around.
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Many believe Ronaldo's inability to press has made it easier for teams to play through the Red Devils, as they lack movement at the top end of the pitch.
Others, however, have highlighted the forward's ability to deliver when it matters - scoring crucial Champions League goals against Villarreal and Atalanta.
Speaking earlier this month, Rio Ferdinand said: "It's blasphemy, when I heard some people talking about Ronaldo’s the reason why Man Utd aren’t going to be a success.
"Ronaldo turning up at Man Utd, and yeah it’s great, but he’s going to be the reason why they don’t do anything. Listen, I’ve been seeing too much of it, don’t talk like that. Please, it’s a disgrace. Absolute disrespect."
Despite Ferdinand's comments, Merson believes Ronaldo's sudden return to Old Trafford cost Solskjaer his job.
"I feel sorry for Ole Gunnar Solskjaer," Merson told Sky Sports.
"He had a plan at the start of the season, having finished second last season. He's chased Jadon Sancho for a long time, got him in, and he's given Edinson Cavani another year.
"You could see his plan: you play Cavani every other game, you have Sancho on one side, Marcus Rashford on the other, you have Mason Greenwood coming in and swapping around, you have pace, power, energy, youth, skill, the lot.
"Then, just before the season starts, he gets given Ronaldo. I think he's thrown it all out of the window. From day one, they're not a better team with Ronaldo in it.
"You don't go 29 games unbeaten away without playing a certain way, and bringing Sancho in gave them another weapon to play that way.
"With Ronaldo up there, you don't have that weapon to play on the counter-attack any more. And Bruno Fernandes was their best player last season - he's hardly had a touch this season!
"It's all snowballed for Solskjaer. He had a plan, and the Ronaldo situation threw it all out of the window, just before the season started.
"They only went for him because they heard Man City were after him. It's not like United were calling Juventus to ask if he's available because Ronaldo is the missing link for United winning the Premier League. He isn't!"
And Merson believes solving the Ronaldo conundrum will be a huge challenge for United's next manager. He added: "It's a problem going forward.
"The fans love Ronaldo, he's the king. Don't get me wrong, he's been one of the best players in the world, but this is the achilles heel.
"The bloke is a legend, with the records he's put up, but that's not today... I think they need to find a position for him, and maybe even give him a free role.
"I don't know if you can keep him in the team, I don't think they're dominant enough in midfield, and without Raphael Varane in defence, they're all over the place."
