Bruno Fernandes credited Alejandro Garnacho for improving his previously poor attitude after the teenager marked a rare Manchester United start with the winner against Real Sociedad.
Garnacho was named United's under-18 Player of the Year last season but has largely remained on the periphery of the first-team squad under Erik ten Hag this campaign.
The teenager proved his worth on Thursday, though, scoring his first senior goal for United with a first-half strike in a 1-0 win at La Real in the Europa League.
While the 18-year-old will hope to feature more for Ten Hag at Old Trafford now, Fernandes says he must continue with a much-improved attitude after failing to impress in pre-season with the senior side.
"He was good but he knows we expect at lot from him. He's still really young. He's doing really well," the Portugal international told BT Sport.
"At the beginning of the season, not at his best, in the tour he didn't have the best attitude that he should have and that's why he didn't get his chances until now.
"Now he's getting chances because he's training better, he's having a better attitude and he deserves his chances."
Garnacho's strike may earn him a share of the plaudits but United still had to settle for second in Group E after Imanol Alguacil's side topped the pool by virtue of a better head-to-head record.
Juventus and Barcelona, both sides to be eliminated from the Champions League, could await United in the Europa League's knockout play-off round, with the draw set to take place on Monday.
Despite not progressing as winners of the group after failing to secure the two-goal win that was required, Fernandes believes the victory will offer Ten Hag's side encouragement.
"Obviously, we knew that we needed a two-goal difference to go through at the top of the group, but it was not possible," he added.
"But we won the game, we keep the momentum, we didn't concede a goal once again, so we have to keep the positives of this game, and there are many positives – much more than negatives.
"So it's just [about] keeping the momentum on Sunday [when United visit Aston Villa in the Premier League]."