The Spain midfielder netted a double, including a superb free-kick, while Javier Hernandez and Adnan Januzaj completed an impressive win at St James' Park in the Premier League on Saturday.
United face Munich in the quarter-final second leg on Wednesday, with the tie level at 1-1, on Mata feels morale will be high in the travelling party.
"Well I think it was a very good day for us, winning 4-0 away in a stadium like Newcastle. It was a happy day for us," he said.
"Great performance, scoring four goals and now we have more confidence for next week in Germany."
The former Chelsea man is cup-tied, but hopes United can produce another famous win at the ground where he lifted the trophy with the Blues in 2012.
"It's frustrating, but I knew before I came that I was cup-tied," he said.
"I will support from home and it's going to be tough, because Bayern Munich is one of the best teams in the world, but we can do it.
"I have great memories from their stadium. I won the Champions League there so hopefully we can get a great result and get through the next round.
Mata has endured a difficult start to his United career, having been asked to occupy an unfamiliar wide role, but he appears to be finding his best form in recent weeks.
The 25-year-old opened his United account in last weekend's 4-1 win over Aston Villa and hopes to play a key role in the closing weeks of the season.
"I'm very happy," he said. "It's been a few very good days for me after my first goal last weekend, so I just will try to carry on.
"I'll carry on until the end of the season, try my best playing at that level and try to score and assist as many goals as I can."