Cotter and Leinster's Joe Schmidt will put their long friendship to one side for 80 minutes of trench warfare at Stade Marcel Michelin where chances are likely to be few and far between.
"This game should not give us many opportunities to score. That is why we need to be very realistic and not miss any chances," he warned.
The New Zealander is guarding against Leinster's loss of marquee players and their sub-standard form in the Heineken Cup and PRO12 League to this point.
Leinster's inaccuracy has been part of the reason why Les Jaunards have been able to build a two-point lead over Leinster in pool five.
He also knows enough to realise that the Irish province are able to hit form for rounds three and four of the European Cup.
"Their season starts in December, with important matches and the return of internationals to their province," added Cotter. "They know that the qualification is on the line now. I think they are willing to put their bodies on the line.
"This is a team that controls the ball well and puts a lot of speed on it. They are used to big matches and this competition.
"With them two points behind us, they know they have no alternative but to get a result here at Michelin. For us, we have to find solutions to counter them by competing in all sectors of the game."
Cotter also feels Schmidt will try to move his larger forward pack around in order to drain energy from their legs and frustrate their close-in driving game.
"I know Joe and I'm sure he will try to move the ball to destabilise us. They will be ready to adapt and they will stay focused and precise.
"We will also be proactive because it would not be surprising to see us preparing new moves, surprises. My players are very excited, focused and motivated to play in the game."