Paul Scholes has suggested Luke Shaw does not take care of himself physically enough, although he hoped the Manchester United left-back still had a future at Old Trafford.
The former Southampton defender returned to the starting XI for the first time in 219 days as United beat CSKA Moscow 2-1 in the Champions League on Tuesday night (5 December) and topped Group A with 15 points.
Shaw, whose relationship with Jose Mourinho has been difficult for some time, looked impressive on his 50th appearance for United, bombing forward at every opportunity in the first half and looking less rusty than expected.
The United manager was full of praise for Shaw, hinting the defender would get more opportunities this season.
"A very positive performance, especially with the balls going forward.
"Creating a lot, great intensity in his game. It was no surprise that in 80 minutes he's feeling the consequences of that intensity; it's normal as it's the first match of the season for him. But he's going to get more
However, according to Scholes, the 22-year-old, who complete the full 90 minutes for the first time since United required extra-time to overcome Anderlecht in the quarter finals of the Europa League in April, is still nowhere near his best.
"I don't think he lives his life as clean as he possibly should do.
"With his body shape, he looks like a player who needs to be playing every week. We had Wayne Rooney, who was a player if he missed a couple of games you knew, he wasn't looking after himself.
"He needs to be playing all the time, Saturday, Tuesday, whatever it may be."
The left-back position has been a source of problems for Mourinho since he arrived at Old Trafford at the beginning of last season, with the Portuguese coach having deployed Shaw, Daley Blind, Matteo Darmian and Ashley Young in the role over the last 18 months.
The latter, who scored two brilliant goals as United won 4-2 at Watford last week, appears to have found a new lease of life in the role and has started every league game since returning from an injury in the middle of September.
However, Mourinho is still looking for a long-term replacement for the 32-year-old and has reportedly identified Tottenham Hotspur's left-back Danny Rose as the ideal candidate, although United would have to pay around £50m for the 27-year-old.
Scholes suggested Young's form and Mourinho's plans to bolster his defensive options either in January or next summer could spell trouble for Shaw, whom he once regarded as the best left-back in the world.
"With Luke Shaw, is he going to get a run of 10 games in the team? I'm not sure, the problem he has got is that Ashley Young has been brilliant at left wing-back," the former United midfielder added.
"I hope there is a future for him. A couple of years ago, I thought we were signing the best left-back in the world."