The on-loan striker has not featured since Boxing Day, when he injured his ankle in the victory over Dagenham.
In the seven games he has been absent, the Shrimpers have won just twice - a penalty shoot-out win over Oxford in the Johnstone's Paint Trophy and a 1-0 win against Chesterfield in the league on Saturday.
Assombalonga will be assessed ahead of the game but is expected to be involved in the squad, while midfielder Ben Reeves could be handed a debut after arriving on loan from Southampton.
Marc Laird and Caolan Lavery are doubts as they battle back from injury.
Midfielder Laird has been sidelined with a hamstring problem while on-loan striker Lavery is struggling with a knock. Both will face fitness tests ahead of the game.