The Brazilian was only just returning from cruciate knee injury suffered last November.
He wrote on his twitter account “No words to describe my feelings at the moment. It is been hard to be positive but I am sure better days will come. YNWA.”
Manager Brendan Rodgers said “It is very unfortunate. He has worked ever so hard to come back and he has been an outstanding player for me and person off the field as well.
“The indications are two to three months. It is quite a unique injury at the top of his right thigh. He has had a scan and we will see a specialist this evening. We’ll see if it needs an operation or whether it is just recuperation.”
Fortunately, Liverpool have strengthened their central midfield options this summer, with Joe Allen and Nuri Sahin arriving. Lucas’ injury could lessen the probability of Charlie Adam leaving the club.