Ole Gunnar Solskjaer's men had gone unbeaten in their previous 13 matches but were well off the pace against the Blades.
Captain Maguire urged the squad to rebound quickly for Saturday's trip to Arsenal, who coincidentally were the last side before Chris Wilder's team to defeat them in the league.
"When you do get a defeat, there's nothing worse than waiting to get back out on the pitch to play and put things right," Maguire told United's club media.
Maguire rued the defensive lapses that he felt proved so costly.
"Not good enough," he added. "We didn't perform to a level we've been playing at. We didn't play at an intensity that we've played at. We haven't created enough to win the game.