Louis van Gaal has admitted that he is not sure of the type of players he needs to sign at the moment for Manchester United and will make signings only to improve his team selection.

The Red Devils have signed two players, Ander Herrera and Luke Shaw, even before the Dutchman officially took charge of the role at Old Trafford. United need to bring in reinforcements in the heart of their defence.

Rio Ferdinand and Nemanja Vidic left United after the end of the last season as free agents. Van Gaal has already admitted that his side needs to strengthen the central defending position.

However, the former Barcelona and Bayern Munich manager says he will not sign players just for the sake of it.

"I don't buy players for the sake of it. I buy players to improve my selection. First, I must see what I need in all the positions from the system we shall play and, at this moment, I don't know for sure," van Gaal said, as quoted by Sky Sports.

United are strongly linked with a move in signing Juventus star Arturo Vidal and the Chilean could make a switch to Old Trafford in the summer transfer window. Real Madrid's Angel di Maria is a reported target, who is also attracting interest from Paris Saint-Germain.

The 62-year-old manager has admitted that he had offers from several clubs before agreeing to join United. Van Gaal says he did not jump on the opportunity to be David Moyes' successor, but admitted the "big challenge" at Old Trafford persuaded him to sign for the Red Devils.

"It was not an instant 'yes' because I had other offers also. So I had to think about it. But I chose 'yes' because of the big challenge," the new United manager said.

"I have been at Ajax – the biggest club in the Netherlands – I went to Barcelona – the biggest club in Spain – and then I go to Bayern Munich, also the number one in Germany. So when Manchester United came I thought I had to say 'yes'," van Gaal concluded.