Haaland, who has scored 42 goals this season, after pulling out of international duty with Norway due to a groin injury.
His return is a timely boost for City's quest to win a treble of trophies in the final two months of the season.
"He trained the last two days really good," said Guardiola. "He will be ready."
City welcome Bayern Munich to the Etihad for the first leg of their Champions League quarter-final just three days after their trip to St Mary's.
But Guardiola said he was fully focused on chasing down Arsenal at the top of the Premier League.
The Gunners have an eight-point lead, but City have a game in hand on Mikel Arteta's men and host Arsenal at the Etihad later this month.
"Bayern is our last thought," said the City boss. "It's Southampton - the Premier League is every day, every week."
Southampton are bottom of the table but shocked City when the sides last met, dumping the English champions out of the League Cup.
City have only won twice in their past five visits to St Mary's and Guardiola is expecting another tough test.
"Now every team is playing for something," he said. "Every game has its own characteristics and you have to be ready, Southampton have always been so tough.
"Always it's been a tough, tough game. Tomorrow won't be an exception."
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