Brighton boss Graham Potter felt they did enough to earn a draw after defeat at Manchester United.
Brighton took an early lead when former United striker Danny Welbeck reacted quickest after his initial header had been saved, nodding in from close range.
However, not for the first time this season, United improved from a lethargic opening period to sweep to victory in the second.
Marcus Rashford levelled when he opened up his body and rolled an excellent shot into the corner from Bruno Fernandes' lay-off, before Greenwood's winner eight minutes from the end.
Later, Potter said: "We contributed well in the game. We took the lead and were probably the team which carried the most threat in the first half - we just missed that final pass.
"Manchester United responded in the second half without creating too many chances. They scored from not too much.
"When it was live, I thought it was a penalty [on Welbeck] then I suppose it's the clear and obvious debate.
"If Mike Dean had given it, I don't think VAR would have overruled it. There's nothing we can do in the end. We're disappointed to come away with nothing."
