The 31-year-old Lion, Cardiff captain, and tight-head prop, suffered medial knee ligament damage during Saturday's Anglo-Welsh friendly victory over West Hartlepool.
Young faces a minimum four-week lay-off - probably eight with rehabilitation - so will also miss the Wales versus Argentina Test at Stradey Park in Llanelli on Saturday week.
The Swansea props Chris Anthony and Ben Evans have been drafted into the squad by the national coach, Graham Henry, who will announce his line- up to face South Africa tomorrow afternoon.
Toulouse not only lost the match and their reputation at Ebbw Vale on Saturday, but they also lost out on a home quarter-final draw in the European Cup. In a dramatic, and often tempestuous, turnaround in fortunes from their earlier 108-16 record thrashing of the Welshmen, they found themselves literally outfought 19-11.
The sending-off of the Toulouse prop Cyril Vencheri for a kick on his opposite number Andy Metcalfe as he lay on the ground was crucial. That came at the end of the first half and eight minutes into the second period the home side picked up a penalty try when the French scrum collapsed in a heap on their line.
The Scottish referee Ed Murray has his hands full all afternoon. Not only did he dismiss Vencheri, he also issued three yellow cards. His handling of the game obviously upset Toulouse and at the end of the match the French international prop Franck Tournaire gave vent to his feelings to the touch judge Ken McCartney.
His manhandling of the official prompted the police to step in and the officers were pushed and shoved as the deflated Frenchman left the field.
One policeman lost his helmet; the whole of Toulouse appeared to lose their heads. Mr Murray was rounded on in the dressing-room area and police reinforcements were called after a club house melee. Ebbw Vale officials have scheduled a meeting with Gwent police today to give a full account of what happened.
The result is that the inaugural European Cup winners missed the chance of a home quarter-final through that defeat and Ulster's battling 23-21 victory over Edinburgh Reivers yesterday. The Irishmen finished top of Pool C and will entertain Toulouse at Ravenhill in a repeat of their earlier match, which they won.
Llanelli's Friday night triumph over Leinster put them in the knock-out phase and Pontypridd also reached the quarter-finals for the first time courtesy of a 32-3 demolition of Glasgow Caledonians.
Neil Jenkins was the guiding light for Pontypridd his boot contributimg 17 points. But also who lighting things up when the floodlights failed was the Wales centre Dafydd James, with a 60-metre try.
EUROPEAN CUP QUARTER-FINALS (rankings after completion of Pool stage in brackets, first named team at home): Perpignan (1) v Llanelli (8); Stade Francais (2) v Pontypridd (7); Ulster (3) v Toulouse (6); Colomiers (4) v Munster (5). Ties to be played 11-13 December.Reuse content