Scans have revealed that Bristol's Dan Ward-Smith suffered a dislocated kneecap and a ruptured patella against Northampton on Saturday.
Ward-Smith has been in superb form for Bristol this season
The 29-year-old, who had been tipped to make his England debut against Scotland, was carried off after just 14 minutes and faces six months out.
"Dan underwent surgery on Monday evening to repair the damage," said Bristol physio Paul Tompkins.
"I've spoken to the surgeon and he's very happy with how it went."
Tompkins added: "Dan's obviously now facing a lengthy spell of rehab but he's a dedicated trainer and he won't shirk the hard work.
"He'll get all the best possible support to help him back towards full fitness."
Bristol coach Richard Hill said on Saturday: "The dislocated kneecap is probably a six-week injury with no complications.
"But if he's ruptured his patella tendon then that's a six-monther."
Ward-Smith, who has been hailed as one of the reasons for Bristol's stunning Premiership campaign this season, is in danger of missing England's two-game tour to South Africa this May and June.
That would also put in jeopardy his place in England's squad for the World Cup in France in September and October.
Ward-Smith was born in New Zealand and joined Bristol from Plymouth Albion in 2005 and represented England A against Ireland at Kingsholm last March.