Forward Maro Itoje will miss England's Six Nations matches against France and Wales with a knee ligament injury.
The Saracens lock was taken off in the second half of England's 32-20 victory over Ireland at the Aviva Stadium on Saturday.
The Rugby Football Union confirmed Itoje would be out for the matches at Twickenham and the Principality Stadium in Cardiff, but is "expected to be available towards the end of the tournament".
Itoje was seen on crutches as England celebrated on the pitch after fears he had picked up a serious injury when he was cleared from a ruck by Peter O' Mahoney.
But although he will play no part against France on Feb. 10 or Wales on Feb. 24, he could be back involved against Scotland on Mar. 9 or Italy on Mar. 16.
Saracens' Nick Isiekwe has been called up to replace Itoje, while head coach Eddie Jones has included four more players in his 35-man squad.
Jack Singleton, Ben Earl, Dan Robson and Ollie Thorley -- all uncapped -- have also joined up with the squad.
Jones is confident that his side won't lose too much momentum despite Itoje's injury.
"Maro's going to be the best lock in the world so it's a big loss, but we've got good depth," he told reporters after the game. "We'll have to fill his spot and guys will just have to work that bit harder to cover his gap for how long he is out for."
Itoje also sustained a knee injury before the November international tests but he still managed to feature in all four matches.
England are level on points with Scotland at the top of the Six Nations table and host France at Twickenham on Sunday.
Information from Reuters contributed to this article.