A fortnight ago, the 26-year-old was omitted from Wales's Rugby World Cup squad prior to the tournament.
The scrum-half suffered a foot injury during his nation's final World Cup warm-up match against Italy.
Now, it has been confirmed by Webb's club Ospreys that the player will be out of action for a "significant" period of time.
The team's medical performance manager Chris Towers told the club's official website: "Rhys sustained a significant foot ligament injury that required surgical stabilisation. He has had the surgery and will be non-weight bearing for six weeks and will then wear a protective boot for a further four to six weeks.
"Only after that will we be in a position to progress his rehab and, at this stage, Rhys is looking at a six to eight month period out of action, a timescale that reflects the severity of his injury."
Meanwhile, Wales are preparing for their second World Cup match, which is against England at Twickenham on Saturday.