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"We are expecting a big battle, especially up-front," he said.
"We did well last year and it set-up a platform for us and even tough we have targeted it again this year we know they will be doing the same.
"They have put in some strong performances and have been working hard so we know it is going to be a challenge."
Johnston has played in all seven of the French side's Heineken Cup games this season but knows it doesn't get any bigger than Saturday's clash.
"You have to be excited, this is what the game is about," he added.
"We all know what is ahead of us and it's going to be a terrific atmosphere in front of 50,000 supporters in a stadium we haven't played in before.
"It's a huge challenge playing away from home but we will have to rise to that.
"We have been watching Leinster this week and they have threats all over the pitch, they are a team that plays a lot of open running rugby like us so it is going to come down to defence on the day."
