The 25-year-old midfielder made his debut in Tuesday's 2-0 home victory over Cardiff after completing his club record £37.1million move from Chelsea on Saturday.
Although Mata was not at his very best, the early signs were encouraging and Young feels the process has been assisted by the welcoming reception the Spain international has received from his new team-mates.
"Juan looked like he had been here for years," said winger Young, whose goal against Cardiff - United's second - came from a pass from Mata.
"He had only been training for two days and it was his first game but he has gelled straight away."
Young believes the United dressing room is a slightly more positive place following Mata's arrival.
He added: "There is a great team spirit here. There are some great characters and the banter is brilliant. I have never experienced anything like it.
"It has lifted everyone; the fans, the staff and the players.
"Obviously, as it is the transfer window, you are wondering who is coming in and what is going on. When you see a player like Juan Mata coming in it is fantastic.
"He is one of those world-class players who can fit into any team.
"When he has fully settled in he will add something special to the squad."