Winger Trimble, who was an important part of Ireland's Six Nations title-winning side last season, suffered ligament damage in his big toe during Ulster's league win over Glasgow Warriors earlier this month.
"Ulster Rugby, in conjunction with the national medical staff, will seek second opinion on the management of the injury, however it is expected that he will be out of rugby for a period of months," read a statement on the club's website (www.ulsterrugby.com) regarding the 30-year-old's injury.
The club also ruled out centre Marshall for six weeks after he sustained a medial knee ligament injury while playing for Ulster Ravens last week.
Ireland host South Africa on Nov. 8 before playing Georgia and Australia on subsequent weekends.
(Reporting By Sam Holden; Editing by Ossian Shine)
