Dimitar Berbatov has argued that Cristiano Ronaldo and Lionel Messi would struggle to perform at a top level in the current Manchester United team.
The Red Devils have had a subpar start to the season and are only two points above the relegation zone after their first eight Premier League matches.
And ex-United striker Berbatov, who retired from football last month, has blasted United's poor management in the transfer window after the departure of Alexis Sanchez and Romelu Lukaku to Inter Milan.
"The winter transfer window is difficult and also on the pitch Manchester United aren't producing as a team at the moment," the 38-year-old Bulgarian wrote for Betfair.
"It doesn't matter how good you are, Lionel Messi or Cristiano Ronaldo could go there and they would have difficulty because the team is not producing as a team.
"When you sell two of your main strikers you need to bring in at least one, just in case.
"Everybody can see that Manchester United need a back-up, Anthony Martial is injured and there isn't another really strong proven goalscorer."
Ole Gunnar Solskjaer, who is under intense pressure, has Marcus Rashford and United starlet Mason Greenwood as his main striking options in the absence of Martial.
But the Red Devils have reportedly agreed terms with Juventus striker Mario Mandzukic, who was linked with a move to Old Trafford in the summer.
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Berbatov would welcome the signing of the 33-year-old Croatian striker and hopes that he could have the same impact as Zlatan Ibrahimovic.
"They have the talent in the young players, but at the same time you need someone like Mario Mandzukic," the former Bulgaria international explained.
"I think he is a really good football player and he hasn't played before in England, but I hope if he comes, he can straight away get into the rhythm of the Premier League.
"Zlatan Ibrahimovic was 34 when he signed for United and he didn't have any problems.
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"Mandzukic is a similar age and I hope he could have the same impact if he signs.
"I think his age is a good thing; Martial, Rashford and Greenwood are all young players, so they need that time and assurance that they don't need to worry and they can learn from the experience of Mandzukic."
United will resume their Premier League campaign next Sunday when they host Liverpool at Old Trafford.
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