The Reds were beaten 2-0 in Russia and must produce their best to make the last 16, but Lucas believes Sunday's 5-0 Premier League drubbing of Capital One Cup finalists Swansea has provided great impetus ahead of the game.
The Brazilian said: "We started the week thinking it could be a really good week for us.
"But we had the defeat to West Brom and then the Europa League loss but we have finished the week off in a positive way by scoring five goals and keeping a clean sheet.
"We know we still have to improve a lot but this victory will give us confidence going into Thursday to try to get through against Zenit.
"With the way we played over there, we created a lot of chances, if we do that again and take them then we'll have a great chance of getting through to the next stage.
"But we also have to defend well on Thursday because if we concede an away goal it will be very difficult for us. We have to make sure we go with a positive and offensive mentality but knowing that if we concede it's going to be very hard."
On the Reds' inconsistency this season, Lucas added: "It's difficult to explain. As players you get frustrated a lot of times as well.
"When you think we are starting to get some good momentum we have a defeat and a setback.
"But there is still time to get that consistency. Our league position is not great (seventh, nine points off the top four having played one match more) but we must keep fighting and see what happens. If we can get that consistency then we can finish this season on a high."