Manchester United were held 1-1 by Young Boys on Wednesday as interim boss Ralf Rangnick blooded a host of youngsters in the Champions League.
With United already assured of top spot in Group F, Rangnick rested all of his key players, making 11 changes from the side that beat Crystal Palace in the first game of his reign.
"I was fully aware before the game that with this team they hadn't played together," said Rangnick.
"We could've score three or four goals but after we conceded the equaliser we were not defending high enough up, not proactive enough and we could also have conceded two or three goals."
Greenwood, 20, was the senior member of a front three also featuring 19-year-olds Amad Diallo and Anthony Elanga.
Rangnick comes with a reputation for trusting youth. He got an early taste of Greenwood's predatory instincts as the striker acrobatically fired home Luke Shaw's low cross just nine minutes in.
Juan Mata, a Spanish veteran at the other end of his career, was handed just his second start of the season.
United fans have been clamouring for Donny Van de Beek to be given a regular run in the starting line-up in recent weeks.
The Dutchman was consistently overlooked by the sacked Ole Gunnar Solskjaer but he did little to force his way into Rangnick's plans with an error for the equaliser.
Van de Beek's intended pass for Aaron Wan-Bissaka was intercepted by Rieder, who then fired into the top corner.
Young Boys' win over United on the opening night of the group had the Red Devils sweating over their progress. They needed a barrage of late goals to take 10 points from their games with Atalanta and Villarreal.
The Swiss champions pressed after the break in search of another famous victory that would have kept alive their hopes of finishing third and reaching the Europa League knockout stages, depending on the result when Atalanta and Villarreal face off in a rescheduled clash on Thursday.
Rangnick showed his priorities were getting as many of his squad time on the field as possible.
The German handed a club debut to third-choice goalkeeper Tom Heaton at the age of 35 and brought on three more teenagers Teden Mengi, Zidane Iqbal and Charlie Savage, son of former Wales international Robbie Savage.
"Quite a few debuts for Tom Heaton, for quite a few youngsters and game time for players who desperately need game time," added Rangnick. "In total I wouldn't say (I was) happy, but it was ok."
Elanga had the best chance to win the game on the counter-attack from Mata's unselfish pass, but the Swedish under-21 international could not beat Faivre with a weak finish.