Harriott and Garvan included in midweek squad.
The match comes too soon for captain Chris Porter, but Macauley Bonne is available after deciding to stay with the U's rather than take up international duty.
And both Owen and Callum have been included in the squad for the game against the Cobblers.
By competition rules, the U's boss is obliged to name either six of the top eleven appearance makers or six of the weekend eleven from the game against Scunthorpe.
The U's venture to Sixfields aiming to make progress in the Johnstone's Paints Trophy and boss Tony Humes is expected to name a strong side.
Recent Colchester United signings Owen Garvan and Callum Harriott are both included in Tuesday night's squad for the game against Northampton Town.
The U's take on Gillingham in their next home game - buy your tickets now at the Saver rate!