It was his full debut for Albion, starting a game for the first time under manager Jimmy Floyd Hasselbaink, and it came at the beginning of their Papa John's Trophy campaign.
A goalless first half saw the England youth international make a couple of good saves and also one cleared off the line by his defender Conor Shaughnessy.
He was beaten in the second half, Jay Bird scoring after a deflection off what looked like a player stood in an offside position but there was no flag from the official. The second came from Harry Darling, he tapped in after Balcombe had parried a shot from Charlie Brown.
Albion nabbed a goal back when Joe Powell sent in a teasing free-kick that MK Don's Matt O'Riley could only steer past his own keeper.
The other teams in Group C of the Southern Section are Aston Villa U21s and Wycombe Wanderers, the latter losing 3-1 to the former. The PJT gets back underway again in October, Villa playing Burton.
Albion's next league match is on Monday night when they face Bolton Wanderers away from home.