Preston loanee Trotman and Chelsea youngster Mellis have been refused permission to play in the competition by their parent clubs.Chris Perry or Wayne Thomas will step into the back four in place of Trotman, while Oscar Gobern could claim Mellis' spot on the bench.
Winger Michail Antonio has this week extended his loan spell from Reading, who have allowed him to play in the competition.
Saints are still without full-backs Graeme Murty (knee) and Lee Molyneux (back), although both are progressing well.
Striker Rickie Lambert, who scored 59 goals in three years with Rovers, returns to the Memorial Stadium for the first time since joining Saints in August.
Goalkeeper Mikkel Anderson and striker Chris Dickson will not be available for Bristol Rovers.
Reading and Charlton have refused the loan duo permission to play, so Rhys Evans and Darryl Duffy will hope to be involved.
Manager Paul Trollope was expected to make changes for the clash anyway, with the Pirates on a five-game losing streak - a run that dates back to their 3-2 win over Saints at the end of September.
Defender Danny Coles could make a surprise return after resuming training following a knee injury, while Chris Lines (shin) is also pushing for a start.
Aaron Lescott will miss out with a groin injury, though, and will be missing for a number of weeks.
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