Toulon stay on play-off course
Giteau, 33, suffered an adductor strain and was replaced by Frederic Michalak.
It was the latest setback for the Australian playmaker whose only appearances since the start of April were the European Cup quarterfinal defeat to Racing 92 and last week's league clash with Toulouse.
Michalak hadn't been named in the matchday squad but he was cool and confident as he converted both his team's tries from Fiji wing Josua Tuisova and All Blacks centre Ma'a Nonu.
Castres only got on the scoreboard in the last minute when Thomas Combezou went over for a converted try.
It was a night of mixed fortunes for Toulon's Australian fullback Quade Cooper.
Just three days after being axed from the Wallabies' Olympic Games sevens squad, he suffered a nervy opening in the rain at Stade Mayol.
First he allowed a pass to slip through his hands and then lost his footing as Geoffrey Palis bore down on him.
Fortunately, neither error proved costly and he made amends on the hour mark when his impressive precision break allowed Nonu to score.
Toulon remain in third place in the table on 72 points, seven behind leaders Clermont and just one back from Montpellier.
"It was a very complicated week with the withdrawal of Drew Mitchell yesterday and Matt Giteau at mid-day today," said Toulon coach Bernard Laporte.
"It's been a black season in terms of our injuries but even with absentees you must always play with pride in this team."
Toulouse outpowered Agen 52-19 in a eight-try mauling to edge ever closer to a berth in the play-offs.
Ugo Mola's Toulouse now sit on 66 points in fifth spot, two behind Racing 92 and five ahead of Bordeaux, with only the top six heading into the play-offs.
For Agen, however, the result meant they joined Oyonnax in relegation to the ProD2.
On Saturday, leaders Clermont made sure of their spot in the play-offs with a hard-fought 16-10 victory over Pau.
The victory left Clermont on 79 points, six ahead of Montpellier, who play Harlequins in the European Challenge Cup final in Lyon on Friday, but whose largely second-string side went down 36-10 at La Rochelle.
Big-spending Racing 92, who face Saracens in the European Cup final next Saturday, also rested their big guns and went down to a 33-27 loss to Brive.
Defending champions Stade Francais allayed relegation fears with a 10-try 69-8 rout of Oyonnax.
In Friday's game, Bordeaux-Begles defeated Grenoble 20-14.
Agence France-Presse
