Derby County forward Rooney detailed the impact Guardiola has had on Sterling's game during an interview with Jonathan Northcroft of the Sunday Times (h/t Simon Bajkowski of the Manchester Evening News):
"Raheem's a player who played on instinct, a lot of ability, and you're only seeing the best of him now. He has worked on it himself but it’s because of Pep Guardiola. Guardiola has got him in a mindset to know the game better and you see a massive difference. It’s knowing the right time to do things and Guardiola is the best at teaching that, in my opinion."
Rooney saw the early years of Sterling's career at the top level firsthand when the pair were team-mates for the England national team. Former Manchester United, Everton and D.C. United attacker Rooney retired from the Three Lions in 2017, just about the time Sterling was learning more from Guardiola.
The winger arrived in the blue half of Manchester two years earlier. He had established a reputation at Liverpool as a player with the pace to stretch any defence beyond breaking point.
There was also a decent range to Sterling's movement, and he had proved versatile enough to play anywhere along the front. Guardiola took charge at City in 2016 and quickly began refining some of the rough edges of Sterling's game.
Efficiency in front of goal began to grow, while Sterling also improved the timing of his runs. In the process, he became a key component of the City forward line, the player who most often helped cap passing moves with the decisive touch.
Sterling's goals helped the Citizens win two Premier League titles in a row. City took the crown in 2018 with a record 100 points, while last season's title was part of an unprecedented domestic treble which also included winning the FA Cup and Carabao Cup.
Team honours were joined by individual accolades for Sterling, who was named Young Player of the Year at the PFA awards in 2019:
While Guardiola deserves a lot of the credit, some praise is also due Mikel Arteta. The Arsenal head coach was on Guardiola's coaching staff until he returned to north London on December 26, but he has been credited with aiding Sterling's development.
Arteta has spoken about fixing the "bad habits" Sterling had learned when it came to controlling the ball, as well as the need to have him drop off markers and create more room to run.
Sterling's improvement in these areas has created a new challenge for Guardiola. He has had to focus his efforts on keeping the player grounded, with the City boss rejecting comparisons between Sterling, Lionel Messi and Cristiano Ronaldo earlier this season.
Sterling had started the 2019-20 campaign in terrific form, en route to scoring 20 goals in all competitions. However, the 25-year-old hasn't found the net in the Premier League since a 3-2 defeat away to Wolverhampton Wanderers on December 27:
A quiet performance during the 2-0 loss in the derby against Manchester United at Old Trafford was the latest example of how far Sterling's form has deteriorated:
It means there is another test of Guardiola's ability to teach. He's helped Sterling take the next step in his development and become a legitimate superstar.
Now the onus is on Guardiola to help one of his best players emerge from a prolonged slump. Those efforts will be crucial amid reports City want to tie Sterling to a new contract and fend off rumoured interest from Real Madrid.