The out-of-favour forward scored the only goal at Withdean, five minutes after replacing Dean Morgan.
The goal had an element of fortune about it. Forster's close-range header in the 64th minute from Glen Little's cross hit his arm and was blocked by Brighton keeper Rami Shaaban before rebounding in off the substitute's chest.
It summed up Brighton's bad luck as Mark McGhee's strugglers suffered with injuries and illness.
Central defender Adam Hinshelwood was forced off midway through the first half with a jarred knee.
Mark McCammon, Brighton's big target man, sank to his knees as the players left the field at half-time and eventually the sick former Millwall striker was stretchered off.
Brighton's misfortune continued early in the second half when they also lost full-back Kerry Mayo through injury.
Top scorer Adam Virgo, having started the match in midfield, went into central defence when Hinshelwood was forced off.
Virgo ended the game as a center-forward, the role in which he has scored seven goals this season as Brighton searched for an equaliser.
Virgo almost pulled Brighton level ten minutes from time with a snap-shot from the edge of the area which the diving Marcus Hahnemann parried for a corner.
Brighton came close to an equaliser again deep into stoppage-time when Leon Knight curled a 20-yard free-kick into the side-netting.