Neil Lennon's side must better Benfica's result in Barcelona on the same night to secure their place in the knockout stages of the competition.
Their 1-1 home draw with Scottish Second Division side Arbroath may not appear to be the best preparation for a crucial European tie but Forrest, who misses the Spartak game with a hamstring injury, believes they will be fired up to perform against the Russians.
"It's good if you've got two games per week. If you get a bad result it means you can rectify it two or three days later by getting a good result in another game," he said.
"The boys are over the weekend game now and hopefully we can take care of Arbroath when the replay comes around next week.
"The boys are full of confidence for Wednesday night and that's the main thing. We've got the squad to play in all the competitions and we know we can cope in them all.
"All the boys have got faith that they can get through to the last 16 and get a good result on Wednesday.
"We've not been in the Champions League for the last few years so it would be a massive achievement."