Louis van Gaal has expressed his first concerns about the state of Manchester United’s squad as he prepares for his opening Premier League match with a worsening injury pile-up and growing anxiety among the club’s support about their lack of transfer activity this summer.

Van Gaal revealed he had already warned the club’s owners, the Glazer family, and the chief executive, Ed Woodward, that the change of manager and implementation of a new 3-4-1-2 system might make it a difficult start to the season but, at that time, he had not factored in the possibility he might have only two fully fit senior defenders for his first game.

His assessment of their squad is that they need to bring in more high-quality players, admitting they are “a little below that level”, though he urged supporters not to panic despite a shortage of defenders meaning he is probably going to bring in two relatively unknown 20-year-olds, Tyler Blackett and Reece James, for their first top-level starts when Swansea visit Old Trafford on Saturday.

Woodward said recently that funds were in place for an extensive period of spending and to “watch this space” but United’s only additions so far came in June when Luke Shaw, now injured, and Ander Herrera joined the team that finished seventh last season, 22 points off the top. Rio Ferdinand, Nemanja Vidic and Patrice Evra all left, along with the second-choice left-back, Alexander Büttner, and that has left them desperately short of defenders now Jonny Evans has been added to their injury list.

Michael Carrick, Antonio Valencia, Danny Welbeck and Robin van Persie are among their other absentees and Van Gaal, explaining why the club had not made more signings, said part of the issue was that he had wanted to give the current squad a fair chance before judging what needed to be done in the transfer market. “I wanted to assess my team. I can judge only when I am the coach and I transfer my philosophy and know how they perform. So that’s what I am doing. I think that is honest. We have to wait and see, because you can buy a player in 24 hours, believe me. That’s not a problem.”

United have been negotiating with Real Madrid about a possible deal for Ángel di María as well as trying to sign Sporting Lisbon’s left-back, Marcos Rojo. Van Gaal would not discuss the persistent links with Arturo Vidal of Juventus – “I’ll discuss it with Ed Woodward not with you” – and said he was relaxed about United’s position. “I am very confident. I am not nervous. Nine players injured and I am not nervous.”

That confidence, however, goes only so far when Van Gaal also says it will be some time before his team are able to challenge the top sides. “I cannot change everything [straight away]. I have to adapt to the culture. It’s a process and we have to make steps. Sometimes you fall and you have to make another big step. That’s the process but you are not champion in October. You are champion in May. I have another philosophy. I have another way of dealing with players to normal coaches. I’m not concerned where we might be [if we have a slow start] and I’ve said that in my meetings with Woodward and the Glazers.”

Van Gaal also revealed he had spoken to Wayne Rooney this week after Tuesday’s friendly against Valenciaout of concern the striker was being weighed down byhis new role as captain. “I said to him: ‘Maybe it was too much for you as captain ... let it fly, I say, let it fly!’ In the US [on pre-season] he scored a lot of goals and made a great amount of assists, so I wanted to say that. “Wayne was very proud to be captain. I asked him: ‘Were you already the captain?’ He said: ‘Yes, I was sometimes.’ But I said: ‘Now you are the official captain’ which is different. I think he wanted to show more than the utmost best to the public: that he is Wayne Rooney, the new captain. And I said to him: it’s more important how you perform, not the captaincy.”