The former United captain believes the young players at the club must be doing more to improve their games and drive the team towards success.
"I don't think any player should have a problem if Ronaldo is making points, obviously, you can tell them face-to-face but sometimes you have to do it through the media," Keane said on Sky Sports.
"But you'd hope these young players are looking at Ronaldo and some of the senior players. The word I would use at any club, particularly the big clubs is, "What's the culture?", "What's it like at the training ground?".
"Are these young players, [Jadon] Sancho, Rashford and [Mason] Greenwood, are they still looking at some of the senior players? You're hoping the senior players are setting a good example. Maybe all of them aren't and maybe some of these young players aren't so quick to do a little bit extra afterwards, and these are all the fine details you have to improve on.
"If the culture isn't right at Manchester United, in terms of the pace of training, the tempo, people's attitudes, the way they speak to the staff, if that's not right, you're going to notice it. To me, there's something missing at Manchester United, there's too many loose ends.
"We're on about Sancho, what's happened to Rashford? Man United have to get to the bottom of it, that's part of the responsibility when you're working with good, young players."
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