Midfielder Hammond and centre-back Trotman made their Saints debuts in Saturday's League One draw with Brentford, but both are cup-tied for the visit of the Blues.
Paul Wotton could step into the midfield while Wayne Thomas should reclaim his place at the back.
Veteran full-back Graeme Murty has resumed training following a dead leg and will be handed a late fitness test.
Midfielder Joseph Mills is close to returning from a hamstring injury but youngster Kayne McLaggon is still recovering from a ruptured Achilles.
Saints ran out 2-0 winners when the teams met at the same stage of last year's competition - when both clubs were in the Championship.