Centre-half Reid is still recovering from knee surgery having been crocked towards the end of last season, and striker Gillespie will not be risked due to a hamstring problem.
Lambert will consider tinkering with his starting line-up, although he is unlikely to make wholesale changes following Saturday's stunning 7-1 demolition of Norwich.
Kevin Lisbie scored twice on his second debut for the Us and should keep his place up front alongside Clive Platt, who also grabbed a brace.
Mark Cousins may get a run-out in goal ahead of new signing Ben Williams, who started on Saturday, while David Perkins, Kem Izzet and Scott Vernon will come into Lambert's thinking after substitute appearances at Carrow Road.
Colchester 4/5, Draw 12/5, Leyton Orient 11/4