The reverse has all but ended their Champions League qualifying hopes.
Spalletti said: “We didn't do well covering Domenico Berardi and their wingers were very wide, so we were always caught half-way and could not get back to recover fast enough.
“We lost a lot of energy that way chasing them down. With time, we found a better shape tactically, but after the second goal the team lost its balance and chaotically tried to get back into the game.
“The result is also down to our own mistakes, because we did get the balls into the box. Sassuolo had the right approach and we compliment them for winning a big game for their prestige. They probably felt comfortable after securing their Serie A safety and tended to lie on the ground a few minutes longer than necessary, but if they let them do it…
“It wasn't down to just one player. After going behind, we had plenty of time to recover and calmly get it back on track. With our possession we would've squeezed Sassuolo and tired them out, but instead we kept losing the ball in midfield and were disorganised.
“At times we were too chaotic and rushed, but we had the time and the balls did get into the box. Andrea Consigli made two or three fantastic saves."