The midfielder is set to undergo an examination with the surgeon who operated on him, in a bid to uncover the problem.
The club's technical and medical staff agree with Janssen that he should not be playing on with the pain.
Last year, the player had a peg and a screw inserted into his leg to help the shin recover.
Janssen looks set to sit out the game against Rapid Bucharest on October 3, and will be keen to find out why the shin is not completely healed.
