The Brazil international recovered from a cruciate ligament injury in time to start the new season but a hamstring injury suffered in August has ruled him out for a further three months.
Lucas told Sportv: "This is the worst year of my career and of my life as well. When we can't do what we like to do and perform your job, it's always a bit frustrating.
"Everything is a bit new to me because I have never had so many injuries. Two in a row is very complicated.
"We tried to stay positive when the first one happened, but the second one was much tougher as it happened so close to the other one."
The 25-year-old moved to Anfield in 2007 from Gremio.