Forster kept a clean sheet in front of Saints boss Ronald Koeman at Staplewood in his first game since patellar tendon knee surgery last March.
The former Newcastle and Celtic goalkeeper had been expected to be out for a year, but showed his reactions remain good with a number of strong saves in the 1-0 win.
Ryan Seager's goal earned holders Saints a place in the last 16 of the Under-21 Premier League Cup.
Source : PA
Source: PA