The Blues have already failed with a bid for Everton defender John Stones but are expected to make an improved offer, with Mourinho confirming he wants to strengthen a backline that conceded the fewest goals in the league last season.
“I am looking for a defender to finish this balance in our squad and after that we are ready to answer anything that can happen to our players,” he told Sky Sports News HQ.
“If one of our players in a specific position wants to leave, we agree a deal for a player to leave and we have to answer immediately and bring in somebody else, because at the moment we have balance in the squad and we cannot lose that.”
As well as a defender, the Blues have been linked with a move for Pedro, who found his opportunities limited at Barcelona last season.
However, Mourinho said: “I have five wingers, if I lose one maybe I am interested in another but at this moment I have balance and quality and numbers.
“It’s a big challenge for us to go through the season with basically the same team and squad from last season.”
One player who will almost certainly not be at Stamford Bridge next season is Mohamed Salah.
The Egyptian joined in January 2014 but struggled to cement a first-team spot and spent last season with Fiorentina.
“I see his future elsewhere,” said Mourinho. “Either on loan or sold with an offer we are happy to accept. We have five wingers and it is better not to have Salah back.”
The Chelsea manager also responded to reports linking Filipe Luis with a return to Atletico Madrid, saying: “If they want him back they have to pay, they have to pay and then we have to react and go to the market to keep our balance.”