Former Manchester United captain Roy Keane says Marcus Rashford "must be ashamed" at his exile under Ruben Amorim.
The 27-year-old has not played for United for seven weeks and manager Amorim has told him starkly that he "has to change" if he is to have a future at Old Trafford.
The England forward has not been included in the squad for Thursday's trip to Romanian side FCSB in the Europa League despite training with the group during the build-up after a period working by himself.
Rashford has been linked with a loan move to Barcelona but time is tight, with the transfer window closing at 2300 GMT on Monday.
Speaking on the Stick to Football podcast, Keane voiced his intense frustration over the situation.
"I can't get involved in the conversation, I still can't understand at any level," he said. "Whatever's going on with players leaving, months left on contracts, a player who doesn't train properly... I don't understand it.
"You might as well leave me out of this conversation. Especially if you think he wants to move, which is more of a reason to train properly so when you go to a new club you're up to speed and when you leave the club at least you showed a decent attitude. He must be ashamed."
Another United great, Wayne Rooney, revealed on the podcast that he had seen Rashford working alone with a fitness coach in front of parents when he took his children to the club's Carrington training headquarters.
"I went into Carrington on Sunday to take my kids to game day and Marcus Rashford was out on the training pitch with the fitness coach, but he was right over towards where the parents were walking past for the kids' games," he said.
"I looked at him thinking how embarrassing it must be for him as parents walk past."
Rooney admitted he had spoken to Rashford and told him he needed to leave the club if he was to resurrect his career.
He said: "I've spoken to Marcus a couple of times and I've given him my thoughts and feelings that he needs to leave the football club because whatever is going on in his life and at Manchester United, he's not the same. For him to have a fresh start, he needs to leave the club."
Rashford, who came through the youth ranks at United, has scored 138 goals in 426 appearances for the club, netting seven times in 24 games this season.
Any potential move is complicated by his reported £300 000-a-week ($374 000) salary on a deal that runs until 2028.