The Blues have been linked with a number of players already this month, including their former midfielder Nemanja Matic, who left Stamford Bridge as part of the deal that brought David Luiz to west London.
However, Mourinho insists any incomings will not include a striker as he has faith in Fernando Torres to continue his good recent form, with the Spain international having scored in two successive games.
"Fernando is calm. He knows we are not going into the market now," said Mourinho.
"In the summer, normally a club like Chelsea is always trying to improve and get new players but he knows that we like him and support him and he knows there is no problem.
"Everyone needs strikers to score goals; the strikers are the goalscorers.
"If you have players in other positions who can score an important number of goals - better. If you have a Frank Lampard - better. If you have Eden Hazard - better. But the strikers are the ones that normally score the goals."
There has been much speculation about the future of Mata, who has failed to win a regular starting place under Mourinho.
The Spanish midfielder has won Chelsea's player of the year award for the last two seasons but has found himself consigned largely to the bench this campaign.
However, Mourinho says both Mata and summer signing Andre Schurrle, who has also spent more time on the bench than in the team this season, will not be sold.
"The others (Oscar, Willian, Eden Hazard), they are playing very well and the team is playing very well," Mourinho added.
"But we need other players. If the game goes on being 0-0 then Mata is coming into the game as he is a fantastic player. But the game went into another direction, we scored and then after that my brain worked in another way.
"But we need them. We need Mata, we need (Andre) Schurrle, we need these players as we cannot do it with three.
"Maybe in February maybe some are not in the best form and Juan is coming at the top of his game."