Michele Politano was again the matchwinner for Sassuolo.
“We didn't play well, especially at the start when it was still XI against XI," Pioli told Sky Sport Italia.
“There were mistakes in the build-up play, but then it was the second match where we had to play practically the whole 90 a man down."
Bryan Dabo was sent off in the opening half-hour.
Pioli continued: “I don't want to find alibis, because the overall performance was not up to scratch. The referee said the first Dabo booking was almost a red, but I don't know what he saw, because the player pulled out of the challenge.
“We should've been more intelligent and sharp, taking a better approach to the game. We were missing important players, but we've been in that situation before and put in different performances.
“Now there are four games left to the end of the season and we'll try to win them all. We have a lot of quality, but we didn't prove that at all today, so will aim to show what we can do against Napoli instead."