The midfielder is unsurprised by the club's spending spree as he was told of the club's rebuilding ambitions when he signed in January 2014.
Spain international Mata moved to Old Trafford from Premier League rivals Chelsea for a fee reported to be in the region of £37 million when David Moyes was in charge.
United have signed 10 players since Mata's arrival and have continued to spend ahead of the 2015-16 season with the likes of Bastian Schweinsteiger, Morgan Schneiderlin, Matteo Darmian and Memphis Depay all arriving in Manchester.
The pressure will be on manager Louis van Gaal to deliver a first trophy since Alex Ferguson's retirement and attacking midfielder Mata is aware the team needs to match those expectations.
"They told me 'you are very important, you are one of the first'," Mata said in an interview with BBC Sport. "I knew the club was going to have these kind of signings.
"It's not just about bringing good players in. It's about playing well and winning trophies.
"That is the pressure this club always has. We have to be fighting for titles. Last season we didn't win any trophy. This season we want to win an important trophy."
