Harwood-Bellis was officially with City for 16 years, making eight senior appearances.
But he made more than 150 appearances during loan spells with Blackburn Rovers, Stoke City, Anderlecht, Burnley and Southampton over a four-year period.
His season-long loan spell with Saints became permanent after Russell Martin’s side won promotion last season.
Asked how he felt the 22-year-old was progressing - as well as the threat posed by Southampton -, Guardiola said: “I’m so happy for him because he’s a lovely guy.
“I’m happy that he’s settling in the Premier League. He’s an incredible threat for Southampton from set pieces and a composer when they have the ball because the manager insists the way they have to play, they have to be close, to move differently,
“They have a different way of playing through the lines. Aaron Ramsdale was magnificent with the ball.
“How good they link inside and lateral balls. I showed many images to the team and warned them. I said: ‘Guys you have to feel that at the right moment you will do good things with the ball and you have to be strong and you have to continue because they are good.’”
Watch part one of the post-match press conference by playing the video above...
