Jose Mourinho wants Luke Shaw to fight for his place at Manchester United and is "testing" him, former United defender Rio Ferdinand has said.
Shaw was left out of the squad for the 4-0 win over Crystal Palace at Old Trafford on Saturday.
The 22-year-old also missed out on a squad place for the games against Southampton and CSKA Moscow after making his return from an ankle injury as a substitute in the Carabao Cup win over Burton Albion.
Mourinho has told left-back Shaw he wants him to "improve," and Ferdinand told BT Sport: "It looks like he is testing them mentally.
"Hit them with something -- are you going to stay down or are you going to get up and fight? If you get up then he says yes, that's the type of man I want in my team.
"As players, that's what you want in your team. You don't want players who will get one hit and shy away from things.
"One bad pass in a game, shy away from taking the ball again for the next 10 or 15 minutes... that's not how you win titles.
"You need players who are going to be brave receiving the ball, not just physically but mentally, and saying whatever happens here going forward, I will be with you guys shoulder to shoulder."
Shaw became the world's most expensive teenager when he moved to United from Southampton for £30 million in 2014.
He was voted the Premier League's best left-back in 2013-14 but has only made 20 first-team appearances since Mourinho took over from Louis van Gaal in 2016.