Mourinho had previously said there was only a remote chance of any incomings or outgoings at Stamford Bridge in the January transfer window, but the Chelsea boss says he will require cover if one of his players departs.
"The window is open and when the window is open anything can happen," Mourinho said.
"We don't think a lot will happen for us. Do we have to react to certain things? Yes. If someone really wants to leave and someone leaves we probably have to bring someone in but we are going to do things in a very calm way."
Mata was furious at being substituted in the 53rd minute of Chelsea's 3-0 win, which came courtesy of goals from Oscar, Willian and Fernando Torres.
The former Valencia man, who has started just nine Barclays Premier League games this term, did not shake Mourinho's hand after leaving the field and he then gesticulated towards the pitch, where his manager was standing, from the bench.
Mata was so unhappy that Essien felt it necessary to calm the 25-year-old down.
Mourinho does not want to lose Mata in the window, but he does not want any of his players to be unhappy either.
"I want to keep him. I don't want him to go. That is my opinion, my wish, but my door is open," the Chelsea manager said.
"The club's door is open too so when a player wants to speak to us we are there waiting for them.
"If you are asking do you want the club to sell him, I don't want to."
Essien's future at Chelsea was thrown into doubt earlier this week when his agent suggested that he could leave the London club due to a lack of playing time.
Mourinho, again, stressed he wanted to keep the player, but admitted that he could not offer the midfielder guaranteed first-team football.
"I offer nobody - not Michael, not nobody (guaranteed playing time)," the Portuguese said.
"I understand that players think about national team, the World Cup and playing regularly, but my players have to think of the team before the players. Michael knows that perfectly."