England scrumhalf Alex Mitchell is set to miss the start of the inaugural Nations Championship after suffering a hamstring injury during a squad training camp last week.
Northampton boss Phil Dowson said on Wednesday that Mitchell is also likely to be ruled out of the rest of the domestic club season, even if the Prem leaders reach the title-deciding Twickenham final on June 20.
That match takes place place two weeks before England face back-to-back world champions South Africa in Johannesburg, with fixtures against Fiji in Liverpool and Argentina in Buenos Aires completing the 'summer' leg of their Nations Championship campaign.
England coach Steve Borthwick could now be without his British and Irish Lions scrum-half for all, or some, of those tests.
"He's pulled it and he's done a decent job on it," said Dowson of Mitchell's hamstring injury. "It's going to be a push to get him any rugby for us (Northampton) for the rest of the season.
"It's frustrating and every director of rugby would say that. Steve would say that when people get injured on club time and they're about to go and play in internationals. It's the nature of the beast. These things happen.”
Mitchell missed the final two rounds of this season's Six Nations after pulling the other hamstring.