I 100% sincerely believe that we would have still won the division with a replacement level closer.
But here's the thing. When a closer has to be replaced, he is not going to be replaced by a replacement level player. He is going to be replaced by the 8th inning guy, with the 8th inning guy being replaced by the 7th inning guy, etc. In reality, you are replacing your #7 pen guy with a replacement level player, which makes it pretty much a wash.
This is one of the reasons why replacement level for a closer is so low.