Until the late-season swoon the team was on a 100-win pace from June 1 on. Instead it projected out to 95. And some of that was w/o Wheeler and Seranthony, and with a flat Harper (even though they also played their best baseball with Harper and Segura missing).
But it's not all on Girardi. The team they broke camp with wasn't fully formed. It took the injuries to Didi and Segura as well as the firing of Joe to really allow Stott and Bohm some time to grow. The bullpen churn was constant, both good and bad. Games that were lost by Alvarado and Knebel and Familia early no longer were late (because Alvarado got good and the others got gone). And the trade deadline moves were all significant, Marsh most of all but Robertson and Thor were at least "hold serve" type moves to get them to the end. So there's no alternate reality where this team might have won 100 games this year or last, I don't think, but they certainly wouldn't have fallen out of the division race this season with a better early run.
Had DD decided to fire Girardi in the off-season, of course, Thomson likely wouldn't be the manager either.