There's no reason to overreact, but there's also very little reason for optimism. Now is some time for some good, old fashioned, realism. The Celtics are old and beatup, they don't have an intimidating defense anymore, and they do not have good energy in the 4th quarter, something that hurt them the last 1/3rd of the regular season as well.
Yeah, they've got a puncher's chance. But it's time to be realistic. A lot has to go right for them to beat the Heat.