Both have tasted defeat to Pool leaders Brive but Diamond reckons the back-to-back games against Agen will be key to deciding whether the English Premiership club progress in the competition this season.
"This week especially is the big game in terms of the overall group," Diamond told the Manchester Evening News.
"As before our trip to Bath last weekend, the message to the players will again be about getting back to basics and concentrating on our defence.
"We've just got to keep our confidence and we'll do that by reverting to the basics of the game and getting them right and then let's see where we're at.
"We have got to be switched on from the first minute. We have had a good sit down as a group and talked about how we are going to move on and we have got to play for 70/80 minutes - not just 40 minutes.
"What we're after is consistency and if we can win away again on the back of last weekend it will just build our momentum."