The former Leeds United midfielder has hailed his club and international team-mate ahead of tomorrow’s Nations League semi-final with Holland in Guimaraes.
The pair are with England after winning the domestic treble with City, with Sterling scoring 25 goals and winning the PFA Young Player of the Year.
Delph joined City three days after Sterling moved from Liverpool in a deal worth up to £49m in 2015 and the former Aston Villa captain has seen his development first hand.
“He’s an amazing young man, people forget he’s only 24,” said Delph. “When we both signed he was 19 (sic) and he looked like a little baby.
“When he came to City he had three seasons under his belt and he’s not that young in football terms – but he’s always had the mindset he will be one of the best players in the world.
“I believe he’s operating at about 80 per cent, so if he can push himself a little more – which he will because his mindset is very good – the sky’s the limit.
“I’m speaking from knowing Raheem since he was 19 and seeing his progression. Right now he’s one of the best players in the world.”
Delph, 29, retained his spot in the England squad despite starting just one Premier League game since Christmas.
He added: “It’s frustrating for anyone who doesn’t get as much time as they want.
“I’m quite experienced now, I’m very level-headed and my main focus was to help my team-mates.
“I always talk about being a team player, I’m a really good team player and I had the lads’ back all the way through.
“It’s always such a big deal for me to be selected for my country. Over the last 18 months we have been doing really well.”