The former Real Madrid forward netted a hat-trick in the 4-0 win over Stoke City on Saturday, taking his goal tally for the season up to six in as many games.
However, the Spain international insists he can still do more for the Blues, as he missed two chances towards the end of the Potters clash.
“I am happy with my start in the Premier League but I can do better,” he said, as quoted by the Evening Standard.
“I had two chances at the end of the game against Stoke (but didn’t take them). I also need to improve my physical condition to be as good as possible.
“It’s a very important week before the international break and we need to give our best for these two matches. We have a Champions League match now and we have to keep working.”