"We haven't had a good start to the season and I'm sure Castres will think we are vulnerable," said McGeechan.
"Individual errors have been killing us. But we have only been losing games by the odd score to top teams and we know from the past that we can compete at this level.
"We have stay clear about what we are doing and start delivering on the pitch."
Despite his focus being firmly on Wasps' clash with Castres, McGeechan will also be looking out for eye-catching performances from potential Lions tourists this weekend.
"The Heineken Cup is a great competition because it throws up so many new challenges.
"You have to be open minded in the way you approach it and it will be part of our selection process for the Lions."