Mourinho said United needed new faces in midfield to replace Michael Carrick, who will retire at the end of the season, and Marouane Fellaini, who is expected to leave the club on a free transfer.
However, the Red Devils boss said he had not yet made a list of targets he would like to pursue in the transfer window as he is focused on ensuring that the club finishes second in the Premier League this season.
United hold a three-point lead over third-placed Liverpool, but have a game in hand over their rivals.
"We are going to invest the basic to improve our squad," Mourinho told Sky Sports.
"We need to find a replacement for Michael Carrick. If Marouane Fellaini leaves, we need to find a replacement for Marouane Fellaini.
"But in this moment, I don't even think about next season or the final, I think about Arsenal (Premier League match at Old Trafford on April 29) because we need one point to finish top four, but we need seven points to finish second and we deserve to finish second and we have to finish second."
A second-place finish in the league would be United's best performance since Sir Alex Ferguson retired in 2013, while the Red Devils are also through to the final of the FA Cup where they will face Chelsea.
Mourinho claimed he had not been given enough credit by the media for his two seasons in charge of United.
"When I don't win, you kill me," he said. "Because I almost do that all the time, when I don't do it, you kill me, which is what you are all ready to do.
"I keep saying that I think we should have probably eight, nine, 10 more points than we had. We could finish this season with 88, 89, 90 points.
"So, we lost a few matches that we should do better. Can we improve next season? Let's try."