The severity of his injury is unknown at this time and Reds' boss Kenny Dalglish said he will undergo tests in the next day or two.
"We don't know what it is so rather than speculate we are best waiting until we get the proper diagnosis tomorrow or the next day and we'll let everybody know in due course," Dalglish said. "He needs to get it scanned."
The injury took place in the 66th minute when the 24-year-old was making a seemingly harmless challenge. His knee only brushed against the Spanish attacker and it did not seem to be a contact injury. As Mata recovered, Lucas turned awkwardly then fell to the ground clutching his knee.
The Brazilian international made it back to his feet and limped to the sidelines. Liverpool played the next few minutes with 10 men as Lucas was being treated. He briefly re-took the field before turning to the Liverpool bench and giving the signal indicating he could not continue.
Medical staff came on the field with the stretcher and removed Lucas from the field as he covered his face with his hands.
Lucas was Liverpool's player of the season last year and has been its most consistent performer this term. He is finally earning due acclaim for the thankless work he performs in the midfield.
He is instrumental in starting the Reds' attacks, completing a higher percentage of his passes than any other player in his position.
He is a key component of the Liverpool defense as well. He tackles well, covers for teammates when they venture forward and shields the defense with impeccable positioning.
Jay Spearing may see increased playing time if the injury keeps Lucas out of action.
Review all the action from Tuesday's game in our Liverpool Live Blog.
Photo via LiverpoolFC.tv
