Loan watch: Britt brace beats Millers
Paul Sturrock's Shrimpers beat Steve Evans' Millers 3-0 at the New York Stadium, Assombalonga opening the scoring after 31 minutes before grabbing a second in the 65th.
The Hornet's forward now has nine goals in total for Essex club, coming from just 12 games in the league and cup.
Dave Martin got the third goal for the League Two outfit.
Joe Garner had a less enjoyable afternoon as his Carlisle United side were sunk 4-0 at home to Notts County.
The forward played the full 90 minutes as goals from Jeff Hughes, Neal Bishop, Francois Zoko and Jamal Campbell-Ryce did for the Cumbrians.
Dale Bennett was once again sidelined through injury as Neal Ardley made his dugout debut in charge of the Dons.
Wimbledon were beaten 2-1 by Cheltenham Town, a late Byron Harrison strike scant consolation for the south Londoners.
Finally, Ross Jenkins was also ruled out of Plymouth Argyle's 4-1 victory over Barnet due to a groin injury.
The Pilgrims were victorious all the same as Barnet's day in the national spotlight ended with another defeat despite Edgar Davids' presence.
Former Watford man Collins John departed after 15 minutes through injury, whilst Rene Gilmartin was on the bench for Argyle.