The Red Devils have completed two deals since the 2016-17 season came to an end in what has been a frustrating summer for the Europa League winner so far.
Manchester United signed Sweden international Victor Lindelof in a £30.7m deal from Portuguese champions Benfica last month to bolster his defensive options.
Mourinho managed to convince Belgium international Romelu Lukaku to move to Manchester United in a £75m deal ahead of interest from Premier League champions Chelsea.
The 20-time English champions have been heavily linked with a swoop to sign Croatia international Ivan Perisic but Inter Milan are proving reluctant to sell.
Manchester United are in Los Angeles for pre-season training and the Red Devils will take on Champions League holders Real Madrid in their next outing in Santa Clara.
Speaking to the club’s website about further additions to the Manchester United squad in the summer transfer window, Mourinho said:
“The problem with us is that we have four players that cannot play football now, not even in training, so it looks like our group is big but it is not because [those four players] can’t even train,” Mourinho said.
“The recovery time is really long. We look at Marcos and there is no chance he can play before December.
“Our squad is not so big. Everybody knows, because I said it was the objective, I would like four players and I have asked for four players, and I am ready to go from four to three because the market is difficult.
“Some clubs think about the market differently to others. We are not a club that is ready to buy and buy non-stop. We are not a club that is ready to pay what the clubs want us to pay.
“So I am ready to go from four to three. With three I just give a better balance to the team and the squad, and it gives us better conditions to compete.”
Manchester United finished in sixth place in the Premier League table last season but still won the EFL Cup and the Europa League.
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