"We didn't play anywhere near where we would've liked to last weekend," said Young.
"The performance against Edinburgh will not be good enough in Castres. It's a step-up but we know we are capable of that.
"But I've said all along that I rate Edinburgh, they play some quality rugby, although the problems on the weekend we caused ourselves.
"They only made two clear line breaks - everything else was from our dropped balls or turn overs. Our accuracy wasn't where it has been, especially at the set-piece and going through phases.
"We struggled to get any momentum. When we did get past four phases we either had a penalty or came away with a try. We produced a lot of opportunities - nine line breaks - and we were one pass away from scoring two or three more tries.
"I'm pleased we are creating opportunities but we have to make sure we are accurate and come away with something.
"Castres are typically French side, we know how tough it's going to be, they'll be really physical especially around the rucks and mauls.
"They will pick and go a lot, have big ball carriers, good runners behind and a good goal-kicker. It's a massive challenge but one we feel we can win if we play to our normal standards."
Cardiff went 10 years without a win on French soil but have enjoyed relative success there in recent seasons with wins over Bourgoin and Biarritz Olympique in the Heineken Cup and last year's Amlin Challenge Cup triumph over RC Toulon in Marseilles.
And Young is understandably hoping his side can take confidence for their recent exploits.
"We've done quite well over the last couple of seasons, picked up results and a couple of trophies," added Young.
"I like to think that will give us belief but every time we've got those results we have played well. So we know it's in us but also that we have to perform.
"We can't afford to give them a head-start - it's hard to turn that around in France. The crowd gets behind them and French sides control the tempo very well.
"We have to make sure we control the pitch and tempo. We can't allow them to get a foot-hold on the game because they will gain in confidence and get better."