Maguire usurped Virgil van Dijk as the world’s most expensive defender when he joined from Leicester City and was particularly impressive on his debut against Chelsea when the Red Devils won 4-0 on the opening weekend of the season.
Since then Manchester United have not won a game, drawing with Wolves and Southampton, and losing at home to Crystal Palace, conceding four goals in the process.
Keane, who won seven league titles with Manchester United, is particularly not impressed with Maguire’s turn of pace and has therefore sounded a word of caution.
"I think he's done OK," Keane told ITV.
"We were talking about how good he is on the ball, but defensively I still think he's... I still have one or two question marks over him.
"I always worry about his pace on the turn, I remember the first goal against Crystal Palace where he's not doing the basics - covering round his other centre-half.
"Listen he's 26 years of age now, you're worried if you're pointing these things out to him."
Not everyone agrees with Roy Keane though as Paul Pogba described the former Hull City defender as a ‘beast’ in the aftermath of his debut against Chelsea while Ole Gunnar Solskjaer has expressed his satisfaction with the 26-year-old’s impact so far.
Maguire will welcome his old friends from Leicester to Old Trafford when the Premier League resumes next weekend.
