Jose Mourinho has accused Andreas Pereira of lacking the will to fight for a place at Manchester United following his switch to Valencia.
The 21-year-old moved to Mestalla on a season-long loan on Spanish deadline day after United granted him permission to leave at the last moment.
Pereira, who spent 2016-17 with Granada in LaLiga, had been tipped for a big involvement in Mourinho's plans this term after the manager was unable to make the fourth signing he wanted before the end of the transfer window.
And Mourinho admits he was disappointed that Pereira decided to leave, although he underlined that a clause in the agreement means they could bring him back to Old Trafford in January.
He told a news conference: "Andreas Pereira was a personal decision, a decision I don't agree with and that I don't think is the best for him and it disappoints me a little bit, because I feel he has the potential to be here fighting for opportunities.
"It can be considered a decision of a young player who wants to play every weekend, but also of a young player who is not ready to fight for something difficult.
"You have to be ready to fight for a position and I'm disappointed he left. But I opened the door, he has a long-term contract, we have an option to bring him back on the first of January, so it's not a drama. But it disappoints me a little bit."
United resume domestic action on Saturday against Stoke City, having won all three of their opening Premier League matches without conceding a goal.
However, Mourinho expects a difficult match at a ground where United have not won since 2013.
"It's always difficult. It's normally not easy for anyone to play against Stoke," he said.
"It's probably because in the last years United was not strong enough. Last season, we didn't win because we didn't play well enough.
"They are aggressive, defend with a lot of people, they are dangerous on the counter-attack, at set-pieces. Hopefully we can be better than in previous years."