Sassuolo and Cagliari are two completely different football teams, can't evaluate SMS' importance to the team on the basis of one opponent. On top of that, we'll be fitter by the time we play Sassuolo but we'll have the Derby on our mind so there are other factors to consider.
Like I said before this game, as much as we could say SMS will be missed against Cagliari - and he was - he wasn't good against Torino and didn't make the difference when we played a similar opponent with a similar philosophy or strategy (Chievo). We can't say we'd have won this game with SMS - I think it's quite likely we'd have got the same result, especially as SMS isn't 100% fit at the moment anyway.
Inzaghi said it himself - SMS' is important to the way we play so his absent is/was felt, but he's not a decisive player.
And Sile is right - we need to improve in a lot of areas to break down teams that set up like Cagliari did. Many times we just inject more pace and work harder and we grab a result like we did against Crotone, for example. But I never felt that was happening yesterday and looking at our subs, I don't think Inzaghi felt we'd get over the line. We just didn't have enough about us and I think Cagliari were very, very good.