The Saints keeper had been sidelined with a serious knee injury he sustained in March last year before returning to Ronald Koeman's starting XI for the midweek clash with the Hornets.
Forster had little do to in what was a dominant performance from the south coast club but his boss was over the moon he was able to come back and keep a clean sheet in what was an important triumph for the Saints.
Southampton also welcomed back striker Graziano Pelle, who came off the bench late in the piece after a knee problem, and Koeman felt the returning pair have made his squad deeper ahead of the season run-in.
"We have a strong squad, and we have some competition," Koeman said.
"We've played a lot and sometimes you need fresh people on the pitch, and that means we are strong.
"They [Pelle and Forster] are very good players and they bring a level to the team.
"When you're a goalkeeper who has been out for nine months, you start in a difficult situation, but you win and you keep a clean sheet, that's amazing."
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