The 3-1 loss at St Mary's dealt a huge blow to second-placed City's hopes of retaining their Premier League crown as United took advantage with a win over Everton to move 12 points clear.
Defender Zabaleta, 28, admits City were deflated by last Saturday's result but says he and his team-mates now have a much more positive mind-set that they can carry into Sunday's FA Cup fifth-round tie with Leeds and then the ongoing title race.
"We had a great session on Wednesday afternoon and the feeling among the lads is very positive," said Zabaleta, quoted on City's official website.
"Sometimes after a game like Southampton you need a few days to think about what went wrong and why, and then come back stronger and I believe that's what will happen.
"We will learn from this defeat and use the deflated feeling we had afterwards as an inspiration to maybe try and rediscover the sort of spirit we had last season.
"We need to stand together, battle and show what we are really capable of.
"We will aim to get back to winning ways at the weekend in the FA Cup and that is our immediate focus now."
The odds are now stacked against City in the title race after last Saturday's game, which Zabaleta believes was the team's worst performance for two years, but the Argentine still has hope they can force their way back into contention over the remaining 12 games.
"We know we gave probably our worst performance for two years or more last Saturday and it couldn't have come at a worse time because it was such a vital game in the title race, and United keep winning so the gap is even bigger than before," added Zabaleta.
"But you never know in football."
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