The West London outfit, who added the FA Cup and Champions League trophy to their cabinets last season, have already succumbed to defeat in a 3-2 loss to Manchester City in the Community Shield.
"Our main challenge this season is to win the Premier League," Mata told Sky Sports News. "It won't be easy because the big clubs have reinforced their squads, and I expect a hard campaign.
"Chelsea have worked well in the transfer market and now it is important for the new men to settle quickly. The defeat in the Community Shield was painful but now it is the start of the season and we must start quickly."
The Blues will take on Wigan Athletic in their opening game of the season on Sunday.