Defeats to Leinster and Northampton have left them with a tough ask to qualify but ahead of the trip to southern France, Tandy was in positive mood.
"Facing the French champions back to back is always going to be a tough ask but it's an exciting challenge," he said.
"You do look at the seedings and wonder how the French champions can be fourth seeds in the pool but we can't dwell on things like that, we've got to just get on with the job.
"Obviously we've been hit by injuries again but that's part of the challenge as a region. With players unavailable it provides opportunities for others and we've got boys who have put their hands up in recent weeks and who have earned the right to be involved."
Tomorrow's fixture sees Wales internationals Alun Wyn Jones, Dan Biggar, Justin Tipuric and Ryan Jones return to the starting XV with Aisea Natoga making his European bow on the right wing. Wales hooker Richard Hibbard settles for a place on the bench alongside Wales Under 20 prop Dan Suter.
Ashley Beck and Adam Jones are out injured while Ian Evans has remained at home with his wife expecting to give birth imminently.
"Castres are back up at the top end of the table so their quality is absolutely clear," said Tandy.
"After a disappointing opening two rounds we know we've got to put in some excellent performances over the next two weekends. We know there's going to be a great atmosphere there as is always the case in France but we've got to stand up and be counted.
"There's a great attitude and spirit among the group and we need to show the utmost professionalism.
"With back to back fixtures, Castres will be at the Liberty next weekend for round two and that is going to be a great evening for everybody involved with the Ospreys. Hopefully we'll have a big crowd of our own to get behind the boys."