Goals from Adam Lallana, David Connolly and Oscar Gobern kept the Saints one point ahead of third-placed Huddersfield, with Adkins' side now prearing for Monday's game in hand at Plymouth.
Forster said: "You look at a team like Southampton, and there's no doubt they are a Championship side and we were beaten convincingly, but that doesn't discount the fact that I was disappointed with the first half where I thought we didn't keep possession well enough or get tight enough on them.
"Goals change games, and if we'd scored the penalty in the final minutes, then who knows? What I will say, and this is not an excuse, is that we have many key players out and only two fit strikers to choose from, so we are running out of bodies.
"I was very pleased with the way we responded in the second half and we pinned them back for reasonable spells.
"The fact is we've played Brighton and Huddersfield here and matched them, and the players have been fantastic, so I'm not about to start being critical at this stage of the season."