Speaking to the London Evening Standard he named Pedro Obiang, Mark Noble, Simone Zaza and Jonathan Calleri as impressive.
Slav pushed those players available to him through a tough training regime as he attempted to iron out the faults he has identified.
He said: “We used the time to work on fitness and I am really happy with the way we have trained, with players such as Pedro Obiang, Mark Noble, Simone Zaza and Jonathan Calleri.
“We had many players — about 13 I think — who were away with their countries but we trained hard with those who were still with us.”