Mata made his debut in the 2-0 win over Cardiff City on Tuesday night and Young said that the Spaniard played like he had been at the club for much longer than a few days.
"Juan looked like he had been here for years," Sky Sports News quotes Young as saying. "He had only been training for two days and it was his first game but he has gelled straight away.
"There is a great team spirit here. There are some great characters and the banter is brilliant. I have never experienced anything like it. [Signing Mata] has lifted everyone; the fans, the staff and the players."
United are currently seventh in the Premier League.