Robben is being offered a bumper £90,000 per week to stay at Stamford Bridge but Real are preparing a big money bid including image rights that they hope will tempt their long-term target to the Bernabeu.
"I would be a lot happier if they were here," the German tactician said. "Getting Robben here is very important for us. That would allow Robinho and [Gonzalo] Higuain to play on the right.
"I want a lot of mobility there, similar to what Zidane gave us."
Drenthe keen to make Madrid move
But Chelsea are placing a £25 million price tag Robben, with Real's valuation closer to the £20 million mark.
"It could be expensive," Schuster conceded.
"We have to move with caution. We need to be prepared to make signings until the final day of the window.
"I am sure that we haven't finished adding to our squad yet."