Milan won 4-2 on Saturday.
Pioli said: “I didn't like that we conceded the first goal very quickly, we must have a better approach.
“Then we played good football for 30 minutes. We faced a dangerous opponent; I saw many positive things, but also many things to work on."
Ante Rebic scored a decisive brace on the day.
“Rebic can help us a lot; he is a strong player," said Pioli. “He has the characteristics to be an important player for us. Too often, strikers are judged by goals, but Ante is giving us even more by helping the team."
Brahim Diaz also struck.
“Brahim has worked a lot. He has made a step forward, he believes in our qualities," said Pioli.
“We conceded two goals from set pieces, we were distracted when they scored the first goal," the coach continued.
“When we work well in attack, then defenders benefit from it too. We can keep the defensive line high or sit back, but we made some mistakes and we need to work on them."