Liverpool have announced Martin Kelly needs a knee reconstruction.
The 22-year-old limped off late in Sunday's 2-1 defeat at home to Manchester United after falling awkwardly and appearing to twist his right leg.
He has had the injury assessed and it requires surgery, which is likely to sideline him for at least six months.
"Martin Kelly is likely to be out of action for a significant period of time after scans revealed he suffered an ACL rupture of his right knee during the match against Manchester United on Sunday," said a statement from Liverpool.
"The England international will now require ACL reconstruction."