Garnacho has struggled for form under Ruben Amorim and was left out of the squad when United stunned Manchester City at the Etihad Stadium in December.
The attacker has since started only three Premier League matches and was linked with a move away from Old Trafford last month amid interest from Napoli and Chelsea.
But he ended up staying in the north west and now faces a battle to secure a regular starting place under his Portuguese manager.
Red Devils icon Keane was in the ITV studio for the FA Cup showdown between United and the Foxes. And the former Republic of Ireland midfielder insisted: “I think it suits him when he comes off the bench. I don’t think you can trust him when he starts.
“When he comes on and the game is stretched, that’s when he comes into his own. They put Leicester on the back foot and the subs made an impact. That’s what Garnacho does well.”
Ex-Arsenal striker Ian Wright then added: “I am very surprised Garnacho did not start in this game. He did the same against Liverpool. He comes on and you can’t deal with him. He has pace and attacks.”
Napoli reportedly presented United with an offer of £40million in exchange for Garnacho in mid-January. But the Red Devils demanded a transfer fee of £60m to sanction his departure, with the ace ultimately unable to secure a move away.
And when quizzed about how close Garnacho came to leaving the club, Amorim explained on Friday: “That is not important. We want players like Garnacho, he is really important. He changed the game with the runs behind.
“He doesn’t need a big change. We are talking about the standards, just to train well, receive the information, understand we want to help him, we want to play in a different system, but it’s not complicated.”
