Sergio Aguero feels knee surgery has given him a fitness boost that allows him to carry out Pep Guardiola's pressing instructions more thoroughly.  Aguero, Manchester City's record goalscorer, has not always seemed to have the best relationship with Guardiola, who has preferred Gabriel Jesus at times throughout his spell in charge.  But the striker has started all 12 Premier League games this term compared to Jesus' two, scoring eight goals - including his 150th in the competition - to lead the race for the Golden Boot.  And Aguero credits the operation he had prior to the World Cup as a factor in making him sharper and more able to follow Guardiola's tactical plan.  "It all comes from my discomfort in my knee. I had been suffering for a long time and had a lot of medical treatment with the physio here at the club," Aguero told Sky Sports.  "I had some discomfort, and then at the end of last season I decided to go for surgery before the World Cup. Since then, I feel great, I don't feel pain at all, and that's probably why I'm enjoying it more and feeling fitter.  "Luckily I am doing well and Pep is happy. I try to do the things he wants and help the team. I'm obviously the first guy who starts the pressing and [the knee surgery] is why I can do it more often."  City have made a brilliant start to their title defence, last term having finished 19 points clear of Manchester United, who were the last team to retain the Premier League in 2008-09.  Guardiola's side are two points