Jose Mourinho has told ESPN FC that he is not happy with Manchester United's transfer window and is ready to accept signing just one more player before the Aug. 31 deadline.

Mourinho has added £90 million forward Romelu Lukaku from Everton and invested £30.75m in Benfica defender Victor Lindelof since the end of last season, but efforts to sign Inter Milan winger Ivan Perisic and a top-class defensive midfielder have so far proved unsuccessful.

Sources have told ESPN FC that United are within £5m of meeting Inter's valuation of Croatia international Perisic, who has flown to China on the Italian club's preseason tour.

Interest in Chelsea's Nemanja Matic and Tottenham's Eric Dier has failed to result in a breakthrough, however, and Mourinho says he may be forced to settle for just one more new signing this summer.

"We always want more," Mourinho told ESPN FC in an exclusive interview. "We always feel there is room to improve, and I cannot say that I am happy with our transfer window.

Man United manager Jose Mourinho has been left frustrated with a difficult summer window. Getty

"What I can also say is that it is a difficult transfer window, and I don't blame anyone -- it is just a reality of things. The market is going in such a direction that many players are difficult to get, not to say impossible.

"Every club is powerful, and they don't want to sell or their demands are really high, so the market is difficult."

Mourinho said his objective is still to sign two more players, but even if they arrive before the end of August, the United manager says they will have lost crucial preparation time in preseason.

"Everton started really strong in the market without the Lukaku money," Mourinho said. "AC Milan, without selling players, are buying nonstop, so it depends on the moment, the objective, maybe on new owners.

"What I will say is that, a long time ago, we thought about four players to strengthen our squad, but it is really hard to do it. If we do the third and forget the fourth, I would be quite happy with that because I know it is difficult."

Mourinho said he knows Lukaku and Lindelof well -- he managed Lukaku at Chelsea and has watched Lindelof play in the Portuguese championship each weekend.

"I am happy with them, and I know that they will succeed at the club.

"The market ends on Aug. 31, so there is plenty of time, but obviously, the objective for me as manager is always to have the players as soon as possible and try to coach them in the team and develop the team with them, and Lukaku and Lindelof arrived to train in time for day one."