Gaston Pereiro struck both goals for the visitors, with Jose Luis Palomino scoring for La Dea.
Gasperini said, "Our first half was one-way traffic, but we played badly with no creativity and never really troubled Cagliari with our finishing or the final ball.
“After the break, with Musso's red card, we lost our shape and did get an equaliser, but above all proved we have problems in various roles. We need to get our heads back on track and resume focus on being a team, because this performance was too ugly.
“We've got to get organised and get back to being a united team. After all that, the first goal was handball, I don't know how VAR evaluated it, there's not much I can say.
“Clearly, there are some matches at home like today where the opponents put up barriers and we need to break them down. However, we made too many mistakes, too many misplaced passes and that made it even tougher.
“This match uncovered some frankly excessive issues and we need to focus less on the table and more on our performances.
“We can work in training on the passing, the touch, but above all the positions. This was a really bad performance, it was lacking in quality and too many players weren't on the right level. It was not an attitude issue, it was one of quality and errors.
“We tend to have some really big mistakes, like the red card today, and those ruin everything when there would've still been time to make up for it."
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