How old are you?
If you're on the younger side, the Blarney Stone on Samsom St is a decent bet. It's a bit of a dive, but it's a heck of an atmosphere. I wouldn't recommend this if you're too much over 25, though. Cavanaugh's is another fun college sports bar in the same area. I liked McFadden's, as well, but again, sort of the same crowd.
Chicky and Pete's is a big local sports bar, but it's definitely not Boston-centric.
The New Deck might be a good bet. It's yet another Irish-centric sports bar, and it's not quite as much of a dive.
This will be an interesting series. Most Philly fans grew up hating the Celts, but for whatever reason, they hate Kobe, too (despite him being from Philly). Maybe they'll just be throwing batteries at the TV no matter what happens.
Please keep in mind, I don't go into the city much (I live in South Jersey right now), and other than McFadden's and Chicky and Pete's, I've only been to each play like once. Still, it's better than randomly picking a place out of the phone book.
My advice would be to go down around Sansom / Chestnut and 39th, and just start going into random places until you find something you like.