Southampton crashed to back-to-back defeats after losing 1-0 at Arsenal on Wednesday.
Consecutive losses has seen Ronald Koeman's men slip to third in the Premier League standings, just a point clear of Manchester United after of Monday's clash with Louis van Gaal's men at St Mary's.
However, Forster is adamant Southampton's togetherness will see the club return to winning ways.
"It was very disappointing, but we take the positives and we move on," Forster said of Southampton's defeat against Arsenal.
"They've got top quality players. From back to front we worked hard and all did our best for the team, but were probably just missing that little bit of luck that you need in football.
"Obviously, it was cruel to concede late on, but there was a lot we could take from the game and if we play like that on Monday night at home it will be a very good game.
"We'll be full of confidence. We can take so many positives from the game at Arsenal and if we can play like that against a lot of teams we will pick up points.
"It's just important we believe in ourselves. The manager does.
"We're so together as a club that it's frightening. It's just important that we hold our heads up high and move on."
History is against Southampton, who have not beaten United in 10 fixtures in all competitions.
