The Blues have already added German midfielder Marko Marin and Belgian striker Eden Hazard to their armoury at huge expense, while Oscar appears to be bound for Stamford Bridge in a £25million switch from Internacional.
The Scot said: "I think the owners have got their enthusiasm back. Having won the Champions League, there's more expectation on them now.
"That was the trophy Roman (Abramovich) always wanted to win, so he's invested in the team again this summer. It's a challenge for us, I suppose, and we always respect Chelsea. You just have to wait and see how good these players are."