Andy Farrell has been ruled out of Saracens' Premiership opener against champions Wasps at Twickenham on Saturday.
The former Great Britain rugby league captain, who moved to Saracens in a deal jointly-funded by his club and the Rugby Football Union earlier this year, is still recovering from a toe injury.
Saracens rugby director Steve Diamond had hoped to include Farrell in one of the club's pre-season friendlies against Bedford or Harlequins, but his eagerly-awaited union debut remains on hold.
"Obviously, we had hoped that Andy would have fully recovered from the problem, but he is not quite ready yet," said Diamond.
"We hope he will be able to return to full training soon, and he may be in contention for our game against Leeds at Vicarage Road on September 11."
Farrell's fellow union convert, former Bradford Bulls wing Tevita Vaikona, is also currently injured, while back-row forward Taine Randell misses the Wasps clash because his wife is due to give birth this weekend.
England Under-18 head coach John Fletcher insists his young side will be better for the experience after beating Italy in the bronze final to round out their 2015 Rugby Europe U18 Championship campaign in style.