United suffered a 2-1 defeat to Sevilla in Germany to end their hopes of a trophy this season as they lost a semi-final for the third time this campaign and Maguire has said that falling short won't be tolerated at the club.
The England defender told BT Sport: "Losing is not acceptable. Getting to semi-finals isn't acceptable."
The Red Devils missed numerous chances against the Spanish side and were beaten by Luuk de Jong's late goal and Maguire felt that United were hard done by.
He said: "The best team lost. They punished us for missing chances. We conceded from two crosses which is not good enough.
"We worked our socks off and deserved to win and get through. We fell short in a semi-final for the third time this year.
"Perhaps that bit of inexperience cost us - we shouldn't have conceded that second goal."
Manager Ole Gunnar Solskjaer admitted the defeat was "hard to take" due to the level of performance from his players.
The Norwegian coach said: "It's very disappointing, hard to take. We played fantastic stuff at times and created chances but couldn't get the goal we deserved."
Solskjaer didn't make a substitution until the 87th minute and has urged the club to improve their squad depth to improve next season.
He said: "We need to strengthen our squad depth of course. I cannot say when or if transfers will be done but we are looking at it. We have to be 100% sure when we make those deals."