The Spaniard arrived at United's training ground for a medical on Saturday morning ahead of a £37m switch to the reigning Premier League champions.
Mourinho has already tied up deals to bring Nemanja Matic, Bertrand Traore and Mohamed Salah to Stamford Bridge this month, and the Portuguese says that Mata's proposed departure marks the end of his transfer activity.
"Hopefully we are done," Mourinho told reporters. "We are not bringing in a striker in this window. We are trying to keep the level and improve, to improve the team, to improve the squad, to change the style of play.
"And we're still trying to get results and still trying to win matches. We need that mixture of young players growing up and experienced players giving stability to the team.
"We are doing that. You can see what we're trying to do. Nemanja is 26, Salah 21, so we are trying to build a team for the future."
The Blues face Stoke City in a fourth-round FA Cup clash in West London tomorrow afternoon.