BT Sport pundit Owen Hargreaves has told 90min that full back Luke Shaw needs to "take responsibility", if he is to make a success of his career at Old Trafford.
A host of injury problems have put paid to the promising start the 22-year-old made to his career at United following his move from Southampton in 2014, and he has yet to re-establish himself after initially breaking his leg against PSV Eindhoven two seasons ago.
His comeback was halted again after suffering ligament damage in his foot in the draw with Swansea City in April.
Shaw has featured very little in the first team this season - his second half showing in the 4-1 Carabao Cup victory over Burton Albion last month was his only appearance - but had travelled with Mourinho's squad to Portugal for their Champions League clash with Benfica on Wednesday night. United won 1-0, though Shaw was not involved.
Former United midfielder Hargreaves, speaking exclusively to 90min, believes Shaw needs to show a desire to make something of his career at Old Trafford, but also appreciated that his manager needed to give him a chance to re-establish himself.
He said: "Jose Mourinho has found a role for virtually everyone at United. Marouane Fellaini has gone from not near the team to one of their most important players when he's fit and healthy. He's made everybody better, but the only one he hasn't is Luke Shaw.
"He (Shaw) also has to take responsibility of that situation. Louis van Gaal has questioned him and Mourinho has done it incredibly openly at times, and it must be difficult for a player. But actually, I think you have to take on that challenge.
"There isn't anybody denying that natural ability but Mourinho demands that competitive side, and I think if United can get him back to what he was then he's one of the best left backs that we have in this country.
"But he has to show Mourinho that he definitely wants it as well. Mourinho wants a reaction out of him, but the only way he's going to get it is if he plays. If he's not playing it's quite disheartening for a young player to train all the time and not have an opportunity to play."
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