England international Sterling scored a hat-trick in Saturday's 3-1 win over Watford.
"He didn't follow his full back two times [against Watford], the second time he dropped him," Guardiola said. "He lost two, three or four balls when he has to avoid it, because he conceded counter attacks and, many simple things.
"Of course I am so glad in terms of what he has done, scoring three goals. I would like to play the first half he played then score three goals, I would be the happiest man in the world.
"But I think it is good for myself too, to feel that we can do better, and I fell the first half was not the best first half Raheem has done this season. That's why I tell him, and we will work on that."