The injury, which blighted the end of his Premiership season, may need surgery over the close season in order for the £9million man to be fit in time for pre-season training.
A club spokeswoman told The Journal: "Jonathan is still having problems with his hamstring and will not be joining the squad.
"He tweaked it a few weeks ago and it has not gone away. We will have to see whether rest solves the problem or if he needs an operation."