Gary Neville admits he is concerned about the number of saves David de Gea is having to make ahead of a season-defining month for Manchester United.
De Gea, 31, has been a standout performer for United this season, which is just as well given how shaky they have been defensively at times.
The Spanish goalkeeper had another busy afternoon against Leeds United on Sunday, making four saves as Ralf Rangnick's side won 4-2 at Elland Road.
De Gea has made 11 more saves than any other shot-stopper in the Premier League this season, proving just how porous the Red Devils have been.
Neville is alarmed by United's defensive frailties and warned they must shore things up at the back if they are to finish in the top four and progress in the Champions League knockout stage.
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"Manchester United do have vulnerabilities. David De Gea has made 92 saves, which is the most in the Premier League, with Illan Meslier, the Leeds goalkeeper, second," the former England defender told his Sky Sports podcast.
"It is not a good reflection on these teams. Manchester United have got to become more difficult to play against.
"They are playing Atletico Madrid away on Wednesday; they have got Tottenham, Liverpool, Manchester City coming up.
"It is a big month and that month is going to define the season in terms of finishing in the top four and progressing in the Champions League."
De Gea's latest string of saves took his tally to 96 for the season at an average of over three-and-a-half saves per top-flight match.
Despite his shot-stopping heroics, he will be disappointed to have only kept six shutouts in 26 games.
Against Leeds, De Gea lost his clean sheet under irregular circumstances as Rodrigo's cross from the left looped over him and nestled in the far corner.
Just seconds later, the Whites scored again through Raphinha, who nudged home from Daniel James' cross, to cancel out Harry Maguire and Bruno Fernandes ' first-half goals.
Although substitutes Fred and Anthony Elanga secured victory later in the second half, Neville has implored his former club to cut out the "mad moments" which must serve as a "warning sign".
"They have got to stop those mad moments, those five-minute periods where they concede two goals, because if you concede two against City, Liverpool or Atletico, you are out of the game," Neville added.
"I think this [the game against Leeds] is a warning sign."
