The striker became the youngest player to start for the Red Devils in Premier League history at 17 years and 223 days old.
Ole Gunnar Solskjaer opted to make several changes for the game against the relegated Bluebirds after losing to Huddersfield Town.
Defeat at the John Smith’s stadium ensured it would be a return to the Europa League for United.
And Greenwood’s presence up front could become a regular occurrence next seaosn for Red Devils supporters.
Solskjaer has already hinted he will undergo major surgery on his under-performing squad, and Micky Gray has highlighted six players who should definitely be on their way out of Old Trafford, and one who may be given another chance.
Among those is Romelu Lukaku, who Gray says has ‘the worst first touch’ he has EVER seen from the Premier League striker and is not the right fit for Manchester United.
Speaking to talkSPORT host Jim White, the former defender said: “Chris Smalling and Phil Jones must leave the club.
“They have done their time, they’ve been good servants, but they’re not good enough to play for Manchester United anymore.
“Ashley Young I don’t think is good enough to play for Manchester United anymore either. I don’t think he’d get in any of the sides from seventh down to tenth.