Tonks had been continuing his transformation from full-back to stand-off for Scotland A in the nation's 16-16 draw with England Saxons when he was forced off the field in the first-half.
He will require surgery to repair the damage, with the recovery expected to take around four to six months.
Head coach Alan Solomons, said: "This is really disappointing news for Greig [Tonks]. He's had a good season for the club and had performed well as injury cover in a new position."
"We all wish him a speedy recovery and look forward to him rejoining the squad at the start of pre-season."
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland