Castres absentees offer Leinster quarters boost
Castres' South African scrum half Rory Kockott runs on the field at the Pierre Antoine stadium in Castres, on December 6, 2013
The news is a boost to the Irish province, who could secure a place in the quarter-finals with a game to spare if they win and Northampton fail to come away from Ospreys with a victory.
Both South African scrum-half Kockott and South African-born France international flanker Claassen missed Castres's last match against Bordeaux-Begles in the French Top 14 due to injury.
"It's still a bit too soon for them," said Castres coach David Darricarrere.
Kockott has a calf problem while Claassen is suffering from a knee injury.
South African back-row forward Pedrie Wannenburg also looks set to miss the clash due to a thigh problem but full-back Geoffrey Palis, who Monday earned a call-up to the France pre-Six Nations training camp, should recover in time from a dislocated thumb.
Hooker Brice Mach and South African prop Michael Coetzee are both expected back from injury but hooker Mathieu Bonello and wing Romain Martial are not expected to return until the trip to Northampton next weekend.