United suffered a shock 2-1 loss to the bottom-placed Blades at Old Trafford.
"It wasn't good enough, we didn't play to the level we have in recent weeks," Maguire said after the game.
"We gave them two goals, now it's time to stand up and be counted because it's easy when you are winning football matches - when you lose it is about how quickly you bounce back.
"We started the game in full control. We have to do more, we made it hard for ourselves, got ourselves back in it and you can't lose it from there.
"We knew at half time the next goal would be crucial, we had great belief we could do that again We weren't good enough tonight and didn't play to a level we expect from ourselves."