that's very far from hate towards him as a player. i just don't think he's the type of guy we're missing. sarri basically said that to fit cr7 and either of dybala and higuain in the same lineup, you need to give up on the flanks and use a trequartista. now, if you're about to start insigne, how the team would look like? 4-3-3 is out of the question as neither of the argies is a winger, and cr isn't one anymore. one could argue that 4-2-3-1 could fit this squad with dybala as the seconda punta and cr as the prima punta, but in that case, you'd rotate cr with higuain, and you'd also need a healthy costa (will never happen) or start cuadrado on the right wing (quite a compromise, considering the other rb's). or you could play a special 4-2-4 with berna/cuadrado on the right, which might be a cool solution - but sarri never played this formation, and isn't considering berna a winger. also, not convinced about insigne's defensive contribution.
tl;dr: even if i put aside that insigne is a prick, i'd rather have a proper am like isco or eriksen than him.