After two defeats in the opening rounds, Glasgow's European hopes are already hanging by a thread. Ryan told the Evening Times: "Without doubt this is an all-or-nothing affair. We have to win tonight and, for me, we need a victory with a bonus point.
"If we can produce that, then Castres will have their own momentum damaged and I think it would make it very difficult for them to progress.
"On top of that, it would provide an outstanding platform to go to France on Sunday week and double up on them.
"But all of our focus has to be on tonight. We have produced in patches over the first two ties, like the first 30 minutes at Northampton. Tonight it is going to be fully committed for the 80 minutes.
"But, by the same token, Castres have four points on the board and they will know that if they can beat us at Scotstoun that will set them up nicely for the return leg at their place.
"It would keep their hopes of catching Northampton in second place very much alive. So I think you could say that for both sides this is a must-win match."
Ryan says Glasgow face a big challenge up front from the French: "Castres have the best scrum in the Top 14 and we have to take them on up front and take that away.
"Our scrum did not work at Munster last Saturday, but tonight it is an absolute must that it is back to its best. It's not an exaggeration to say this is easily the biggest game of the season so far."