José Mourinho says Manchester United have acquired a top player in Romelu Lukaku, but the £75m striker still has to prove himself in big games

José Mourinho says Manchester United have acquired a top player in Romelu Lukaku but the manager admits the striker still has to prove himself at the highest level.

The Belgian was signed from Everton for £75m at the start of last week and scored his first goal for United in Tuesday’s 2-1 victory over Real Salt Lake. Lukaku has registered 85 times in the Premier League since 2012 – second only to Manchester City’s Sergio Agüero – but has never played in the Champions League, as he will next term.

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Of Lukaku, Mourinho said: “We got a big player, a player that I can compare with what he was a few years ago, because he worked with me for a few months [at Chelsea] and his development was very good. We have a player who is now a top player. You have to prove it at the highest level, he has to do it for United, in Champions League matches, but he has amazing qualities.”

Mourinho had an interest in Real Madrid’s Álvaro Morata but the striker is set to move to Chelsea for around £58m. “I’m not interested in what Chelsea FC does, really,” the Portuguese said. “Chelsea it was obvious they were going to sign a striker after the [unhappy] situation with the manager and striker [Diego Costa].”

Ideally Mourinho wants more new players, with Victor Lindelof, a central defender, his other fresh signing so far this close season. These would be an attacking player – probably Internazionale’s Ivan Perišić– and a defensive midfielder, Tottenham Hotspur’s Eric Dier or Chelsea’s Nemanja Matić.

But the manager stated he would settle for one, with Perišić being the most likely. He pointed to the £45m initial fee paid by Manchester City to Spurs for Kyle Walker, a right-back, as an indication of how difficult the market is.

“The transfer fees don’t surprise me, really. I kept saying there is big economic power at every club. It allows clubs to say no. To say pay what I want or you don’t get it,” he said. “I can imagine City would like to pay £25 million for Kyle but they were told this is the price.

“The option is to pay or not. I am used to the club paying big amounts for big players. Now everybody pays big amounts for good players. There is a difference between good players and big players.

“The problem is we have four players that cannot play football now, not even training [the injured Marcus Rojo, Ashley Young, Luke Shaw and Ibrahimović, who is a free agent]. So it looks like our group is big but it is not and the time for their recovery is really long. Marcos has no chance before December as an example, so our squad is not so big.

“Everybody knows because I told I would like four players and I’m ready to go from four to three because the market is difficult, some clubs think the market is different from others. We are not a club that is ready to pay what the clubs want us to pay so I’m ready to go from four to three.”

Mourinho stated that while Rashford can play alongside Lukaku he is not a playmaker. “I don’t think Marcus is a number 10, he’s a pure attacking player,” the manager said. “Not with that quality to play between the lines, and assist, I think he’s an attacking player who can play in a two or one, he can play anywhere and you demand a little bit more defensively, he can play with Lukaku after playing with Zlatan.”

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Rashford is placing more pressure on himself to improve next term on last season’s 12 strikes. “This season I will put more pressure on myself to do so and more responsibility,” he said. “Last season was a different experience. I experienced things I hadn’t experienced the year before. It puts you in good stead for the future. It impacts you quite a lot how you recover from that. That’s what I was looking at. “

United face City at Houston’s NRG Stadium on Thursday night in the first Manchester derby staged overseas. Yet Mourinho played the fixture down. “It’s nice for US and US fans, it’s very good for the event that deserves everything from us because of the phenomenal preparations, I think it’s very good for both clubs and there is always a little bit of rivalry, a bit of an extra ingredient,” he said. “But for me it’s a friendly, I’m not going to jump to celebrate a goal, or cry if we concede, it’s preparation for me, European Super Cup is a match, West Ham is a match.

“But we feel a responsibility to the American people who love soccer and United fans and we couldn’t play City in China but here the conditions are phenomenal, the organisation is phenomenal.”