Manuel Pellegrini's side go into next Wednesday's clash at the Nou Camp as massive underdogs to reach the quarter-finals following a 2-0 first-leg home defeat by the Catalans.
Toure, who played for Barcelona from 2007-10, still retains hope City can progress, although he admits it will be tough for his current side to progress, and he will not celebrate if he scores.
"I never celebrate a goal in the Nou Camp," Toure told radio station RAC-1.
"We have a very complicated (situation) in the Champions League against Barcelona. To me, it will also be very difficult to play at the Camp Nou."
City will go into the clash on a high after lifting the Capital One Cup against Sunderland last weekend, in which Toure scored his side's equaliser in the 3-1 success.
"When I left the club, my ambition was to keep winning titles and I'm achieving this goal. The Capital One Cup was very nice, but the Copa del Rey (with Barcelona was good) too."
Toure is full of praise for Barcelona stalwart Carles Puyol, who has announced he will leave the Catalan giants at the end of the season after spending his entire career with the Spanish champions.
"He is a great person, very important for the club and has an amazing mind," said Toure, who admitted he does not know what his own future will hold.
"I do not know if I will end my career at Manchester City. We'll see how it goes."