The Guinean midfielder sustained an injury in the Champions League defeat at Barcelona.
He will miss the rest of the season and is a huge doubt for the Africa Cup of Nations, which starts on 21 June in Egypt.
"It is unlucky," said Liverpool manager Jurgen Klopp. "A high-grade adductor injury; the tendon is ruptured.
"Bad news for us and him as the Africa Cup of Nations is coming up."
Keita was forced off after 24 minutes in the first leg of the semi-final at the Nou Camp, which the Reds lost 3-0.
He will miss the return leg at Anfield as well as Liverpool's remaining two league games against Newcastle United and Wolves.