Alejandro Garnacho scored his first Manchester United goal but a 1-0 win over Real Sociedad wasn't enough to avoid finishing second in their Europa League group and facing a playoff against a team dropping out of the Champions League.
United needed to win by a two-goal margin in San Sebastian to overhaul the Spanish side at the top of Group E and progress directly to the Europa League last 16.
Instead they will face a knockout round playoff against one of the eight third-place finishes from the Champions League group stage -- Ajax, Barcelona, Juventus, Bayer Leverkusen, FC Salzburg, Sevilla, Shakhtar Donetsk and Sporting CP.
But while Erik ten Hag's resurgent side did not get the desired result, there was plenty to cheer in a maiden goal for their 18-year-old starlet.
"I am really happy with his performance: he was a threat, he scored a goal and did well defensively. It was a great finish, a great attack and really enjoyable," Ten Hag said after the match. "You do not always get what you deserve. In the first game there was a penalty that we didn't think was a penalty, but today we won.
"There are some positives: Garnacho, Donny [van De Beek] back in the team. We scored a good goal, but unfortunately we didn't get the two we needed."
In just his second start for the club, following his full debut in last week's win over FC Sheriff, Garnacho gave another glimpse of his promise with a superbly taken goal in the 17th minute at the Reala Stadium.
There was something fitting, too, in the assist coming from a man who burst onto the scene at Old Trafford as an 18-year-old almost 20 years ago, before Garnacho was even born, Cristiano Ronaldo.
The 37-year-old, starting his third-straight game, picked out the run of Garnacho surging inside off the left flank with a perfectly weighted pass. Garnacho took the ball in his stride before finishing emphatically into the roof of the net at the near post.
The Argentine youngster could have had a second soon after but this time, cutting in on his right foot after being found in a similar position, he slipped as he shot and sent his effort high over the bar.
Just as in their 1-0 Premier League victory over West Ham on Sunday, though, Man United were grateful to David de Gea for preserving their lead.
The Spanish keeper made an outstanding double save shortly before half-time, diving at full stretch to get one hand on a dipping strike from Andoni Gorosabel before quickly recovering and blocking Pablo Marín's follow-up effort over to crossbar with his head.
While the scoreline suited the home side, it was they who had the better chances to score after the half-time interval as they continued to push men forward.
In contrast, despite needing a goal and even bringing on Harry Maguire as a makeshift striker, the visitors failed to muster a single shot on target in the second half, allowing Real Sociedad to finish top on goal differential and reach the last 16 of a European competition for the first time inn 20 years.