His 2017-18 campaign ended with the Champions League defeat against Liverpool on 10 April but he returned for the World Cup in Russia and scored two goals as Argentina reached the last 16, where they lost to eventual winners France.
"During the past few years, I felt pains at times and when last season came to an end, we decided it would be best to start treatment. And the results were very positive," he told City's website.
"Dr [Ramon] Cugat did amazing work with my knee. I'm not sure if this is the best start [to] a season for me, but it's the best I've felt in years. It shows in my game."