The Blues fell to a 3-1 defeat in last week's first leg, but Lampard is confident that they can get the result they need to progress to the last four.
"We have to be positive and try to win the game," Sky Sports quotes him as saying. "We will have to keep our heads because with the squad we've got we can win by two goals for sure.
"But we can't be too gung-ho. It's all about finding the right balance. We can certainly win by two goals. We have to take away that feeling of chasing goals from minute one a little bit.
"It's more about getting our focus and attitude, pacing our game right. If we do that, the consequence could be winning by a couple of goals."
Chelsea will be aiming to win the Champions League for the second time in their history, after beating Bayern Munich in the 2012 final.