England fly-half Owen Farrell may not be fit to return to action until mid-October as he continues to struggle with his back injury.
It is understood Farrell will have a scan to assess the issue on Thursday.
"We don't know if he will play against Toulon [mid-October]. There's a chance he could face Wasps [on 9 October]," said Saracens boss Mark McCall.
Northampton say Dylan Hartley's back is improving with England's captain set to return "in the next couple of weeks".
Farrell will still be named in Eddie Jones' squad for the November internationals on Friday, but will be unable to take a full part in the camp in Brighton next week.
Farrell, who has just turned 25, has not played this season and McCall told BBC Radio 5 live that Saracens "are genuinely not sure" when the fly-half will play again.
"It's really a day-by-day thing with Owen at the moment - it's a bit of an open-ended injury which is always frustrating for the player, for the media, for me," added McCall.
"[England] are clearly aware of the situation. I don't suspect he will do an awful lot [in the England camp].
"He's been able to work on his skill work, the one thing he hasn't been able to do a lot of is his kicking."
Meanwhile, Northampton director of rugby Jim Mallinder says England captain Hartley is on the mend, although he will not feature for Saints against Exeter in the Premiership on Friday evening.
"He's improving, we aren't exactly sure but hopefully in the next couple of weeks he will be playing," Mallinder told 5 live.
"He's been a little bit sore, we are giving him a bit of time now just to get it totally cleared up, and hopefully before those European games [in mid-October] he will be back to 100% fitness."
England start their autumn Tests against South Africa on 12 November, followed by games against Fiji, Argentina and Australia on consecutive weekends at Twickenham.