England fast bowler Chris Jordan will be out for between four to six weeks with a side strain picked up during the one-day series against New Zealand.
The injury rules him out of contention for the start of the Ashes against Australia, which begin on 8 July.
The Sussex paceman, 26, has won eight Test caps since his debut in June 2014 but did not play in the drawn two-Test series against the Black Caps in May.
He was left out of England's 14-man squad for the pre-Ashes training camp.
Jordan was injured in England's rain-hit loss in the second one-day match at The Oval on 12 June.
Sussex cricket manager Mark Robinson also confirmed former England bowler Ajmal Shahzad is likely to miss the rest of the season with a pectoral muscle problem.
The 29-year-old right-armer was out for four weeks with the injury earlier in the summer, before returning for the County Championship game against Durham at Arundel on 15 June.
However, the strain flared up again after the former Notts man's fourth over and he was unable to bowl again during the match.
"We don't think Ajmal Shahzad will play again this summer," Robinson told BBC Sussex. "He felt a bit of discomfort before the Arundel game. The feeling was a bit more stiffness, it was an extension of the injury but in a slightly different place."
|Sussex reliant on Steve Magoffin|
|The 35-year-old Australian has taken 47 County Championship wickets at an average at just over 20 this season.|
With Lewis Hatchett, Jimmy Anyon and Tymal Mills also struggling with injuries, only three Sussex seamers who play regular first-class cricket are fit - Steve Magoffin, Matthew Hobden and Tim Linley, who is on loan from Surrey.
Despite the injury crisis in the bowling department, Robinson heaped praise on the batting performance of captain Luke Wright, who smashed a magnificent unbeaten 111 to power his side to victory against Gloucestershire in the T20 Blast on Friday.
Needing 43 off the final two overs, the opening batsman blasted 34 off the 19th over including four sixes.
"It was a special, sensational innings," Robinson added.
"Wrighty' got stuck but he hung in there and then suddenly he got his timing right and then the rest was history."