James Haskell faces a race against time to be fit for England's Calcutta Cup clash with Scotland.
The Wasps flanker injured his left ankle in last weekend's 24-13 victory over France in Paris, and an MRI scan on Tuesday confirmed he damaged the posterior capsule.
Haskell will miss Wasps' Premiership game against Newcastle on Friday - but if all goes to plan he could still be in contention for the trip to Murrayfield eight days later.
A Wasps statement read: "He has begun a programme of intensive rehab with the aim of being fit and available for selection before England's match against Scotland on March 8."
Haskell, who was hobbling around Wasps' training ground on Tuesday, suffered the damage after his foot was caught at an awkward angle under a pile of bodies.
Initially, he feared the injury would be similar to the horrific broken leg suffered on Saturday afternoon by Arsenal footballer Eduardo da Silva.
Haskell said this week: "My knee went one way, and my upper half went the other - so when I looked down, I thought I might have an injury like Eduardo, and that some bone might be sticking out of my sock.
"With the help of the England medical staff and the Wasps’ medical staff, I hope I’ll be back and in contention for selection."
If Haskell fails to recover, Tom Croft would be his likely replacement against Scotland - after the Leicester loose-forward made an impressive Test debut off the bench in Paris.