The Blues boss believes his side has won the race for the 25-year-old midfielder and trumped their London rivals to the deal, but has dismissed talk linking Mata with a move away.
Spurs looked set to capture the signature of the Brazil international in a move that would see him become their third club-record deal of the summer, following deals for Roberto Soldado and fellow compatriot Paulinho.
However, a late 37 million euro bid lodged by Chelsea Thursday added competition for his signature, and Mourinho now believes the player has chosen a move to Stamford Bridge instead.
"He’s an extra target [to Rooney]. We have to do a medical," Mourinho told reporters when asked if the move for the Brazilian is complete. "I am happy with my squad, but every one of us says we are happy. We always want something more and never close the door."
The 25-year-old travelled to Tottenham’s Enfield Training Centre to complete a medical on Wednesday, before a fee had been agreed with the Russian Premier League side.
Mourinho believes the Chelsea's London rival acted with "danger" by agreeing to a medical before a fee, but denied speculation that any move for the midfielder would force Spain international Mata to leave the club.
"That's the danger of medicals before contracts," he continued, when asked about Willian’s potential move to White Hart Lane. "I want a 22-player squad. If somebody comes, somebody leaves. We don't need to sell. Loan probably – yes.
"He [Mata] is a player we all like, a player we all want to keep. We want more quality, not less quality."