IMO Celts could win first round with no Garnett AND no Powe. They could win second round with no Garnett and healthy Powe. After that they'll need Garnett to be feeling super fine....(though, they might take Cleveland to 7 games even without Garnett)
Powe's complete health is essential if KG is limited.
Without Garnett, opponents let down their guard slightly. The Celts can take advantage of that to some extent. And also be a different sort of team, relying more on guards, etc...could be a very interesting team...but not champs imo..