United needed an extra-time penalty to advance to the Europa League semi-finals.
And Mata was happy to see his team's determination to win despite their fatigue.
Speaking to the club's website, the Spaniard said: "The team is quite tired physically. At this stage of this season it is important to win games like today and we go through. We stay in Germany for quite some time and hopefully we can win.
"Both teams got a little tired and there were more spaces and more chances. They played some good football and credit to them. We could have scored more goals in extra time but we are through.
"I don't think it was complacency. They played some great football in their last game. We scored one goal, of course we are happier when we score more but now it is time to recover and wait and find out who we get in the next round."