United are now just one point ahead of third-placed Liverpool, albeit with a game in hand to play against Bournemouth in midweek, and Mata acknowledges the importance of quickly recovering from the shock setback.
"I think there's no excuses. It was a bad day for everyone and a bad result," he told Sky Sports News. "We're disappointed and we understand the fans' situation. It was a slow game and we didn't play well at all.
"After last week we wanted to keep going with the same spirit. We didn't' create enough chances. They are fighting for survival but there's no excuses.
"Basically we didn't do our job today which was winning. Our aim is to keep second position and it's not done yet. Today was a bad day and we need to move on.
"We had a few days to rest and prepare but football is unpredictable and sometimes things happen. We're sad and disappointed with ourselves."
This was just the second time in Premier League history that United have lost at home to a side starting the day as the bottom club.