Squads in the pool stages have been increased from 34 to 36 and clubs which reach the knock-out stages will be allowed to bring in three additional players.
Bristol, the Guinness Premiership leaders, have been dealt a cruel blow in the build-up to their second derby in the opening three weekends of the season.
They will be without their former England hooker Mark Regan for Sunday's match against Gloucester because of a fractured thumb. He is expected to be out for up to six weeks.
Regan has had to have a screw inserted into the thumb - an injury sustained during Sunday's victory at Newcastle. He played on for 20 minutes before realising how serious the injury was.
There was more bad news for Bristol with Gloucester expecting to have Mike Tindall back in time for the derby at the Memorial Stadium. The England centre has recovered from a muscle strain picked up against Sale at the weekend.
Saracens have ear-marked a Second XV game against Gloucester on 26 September as a possible date for their rugby league convert Andy Farrell to play his first competitive game of rugby union. They hope Farrell will have recovered from a bruised toe by then.
Ahead of their autumn internationals, France have been hit hard with injuries. Most worrying is that to fly-half Frédéric Michalak, who picked up an ankle injury while playing for Toulouse at the weekend.
Benoît Baby also damaged ligaments in his left knee in the same match as Michalak. The centre requires surgery and will be out for up to eight months.
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