The 20-year-old is a target for returning Blues boss Jose Mourinho, with the youngster set to decide his future when his summer holiday is over.
"I gave him a call and told him my opinion. I said he must follow his feelings," Janssen told NUsport. "If he thinks he's ready for the next step, then he must do it. I think he is ready for a next step but the step to Chelsea is very big.
"Eventually I think he will reach that level. Only the question is if he will reach that level already, or if he needs some time to adapt. I think the last one."
Janssen also revealed that the youngster would not be sent on loan if he did join Mourinho's men at Stamford Bridge.
"It's the plan that he'll join the Chelsea squad, learn a lot and play on a regular basis," the veteran explained. "It all looks very positive for Marco but he must make his own decision. I told him that opportunities like this don't come that often and, if you get a chance like this, I think you must do it."