BBC Radio Manchester's Gaz Drinkwater and Joe McGrath expressed their concern for winger Jadon Sancho on The Devil's Advocate podcast.
The former Borussia Dortmund attacker has been left out of Erik ten Hag's side that travelled to Spain for a warm-weather training camp ahead of the return of domestic football so he can continue to work on an individual programme.
But Gaz believes there could be a bigger issue, saying: "I don't actually know what the full ins and outs of Sancho being left out is but it is a worry.
"He is not a winger that is going to chase every ball down and look like he is working really hard and Ten Hag may not like that about him.
"In recent games when he has gone off the boil a little bit in terms of form there have been games where it is evident that the effort is not there."
McGrath is also concerned, adding: "That would be the biggest shame for me since the Fergie era of a player not living up to the potential at United - if Sancho is one of those players that doesn't make the cut.
"We were linked to him for so long and he had glimpses of being a future star in some of the games last year.
"If this is going to be him not having his moment and Ten Hag using other wide midfielders and other attacking options I will be so upset because he had so much promise and I had so much hope for him in a United shirt."