Manchester United legend Rio Ferdinand is concerned centre-backs Harry Maguire and Victor Lindelof don't suit interim manager Ralf Rangnick's style of play.
Although the German coach is only in charge on a temporary basis, he will start a two-year consultancy role in the summer and have a say on the team's future.
Ferdinand believes Rangnick wants his centre-backs to defend on the halfway line and win the ball high, something that could leave Maguire and Lindelof vulnerable.
Both defenders lack blistering pace and rely on their reading of the game to get by.
Ferdinand is also concerned whether Maguire and Lindelof are good enough to win the Premier League. Both players are still waiting for their first major trophy at Old Trafford.
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"I think there can be improvements, Ferdinand told his Vibe with Five podcast.
"I think Lindelof is a good player, a solid player. Is he gonna enable you to win the league? I don’t know. Is Maguire gonna enable you to win the league? That’s the question as well.
"Because the thing about someone like Ralf and all the top teams now, when they play now, they play on the front foot and are aggressive and wanna win the ball high.
"So, therefore, you need defenders who can defend on the halfway line, one-v-one, two-v-two – or three against two if you need that security.
"If you can play two-v-two one-v-one or on the halfway line, that enables you to have more bodies in the opponent’s half of the pitch.
"Hence why teams do that to win the ball early and be closer to the opposition’s goal to score goals.
" Liverpool do it tremendously well, City do it really, really well as well, Bayern [Munich] do it - three of the best teams on the planet.
"Everyone’s going that way of playing football, Ralf Rangnick’s style of football is like that. Can Maguire do that, can you do that with Maguire? That’s my problem, that’s my issue."
Rangnick has previously said he wants to play an intense style of football that would require his defenders to win the ball high.
It is still possible to do this with a slower centre-back, although that player would need someone with pace alongside him in case a forward got in behind the defence.
"Our idea is clear, it’s very, very similar to my coaching friend Jurgen Klopp," Rangnick told The Coaches' Voice last year.
"Our Red Bull football is heavy metal, rock and roll, it’s not a slow waltz. We hate square passes, back passes, just having the ball ourselves doesn’t make sense."
Thomas Tuchel, who was given his first coaching gig by Ralf Rangnick at Stuttgart more than two decades ago, has previously spoken about his compatriot's "forward-defending" style.
"You can say that I am a staunch representative of the Stuttgart football school," once said the current Chelsea manager.
"Just as you are shaped by your parents’ upbringing, you are shaped as a young coach here [in Stuttgart].
"In this respect, I have dedicated myself to the Stuttgart school, which stands for offensive thinking, forward-defending [pressing] and ball-oriented play.
"Of course, I continued to develop - but the foundations were laid in Stuttgart."
