The Merseyside outfit lost 2-0 in Russia last week before they beat Swansea City 5-0 at Anfield on Sunday, and Lucas has insisted that he and his teammates are in a positive mood ahead of Thursday's return leg.
He told Sky Sports News: "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."
Lucas has featured 18 times for Liverpool this season.