Probably the least of the Celtic's problems. Looking at the guys that you'd expect to take a couple free throws per game:
Bradley's pretty decent (81%)
Sully's decent for a big (75%)
Turner's fine (77%)
Bass is good for a big (79%)
Crowder could be a little better (71%), but that's not terrible
Zeller is very good for a center (83%)
The guys you'd like to see some improvement from are KO (only 69%) and Smart (64%). But, now you've added Isaiah Thomas to the mix, and he's got a career mark of 86%.
And if they improved from 75% to... 78% (placing them in the top 5), given that they take 20 free throws a game that's only an improvement of 0.6 pts/game. It's not NOTHING, but it's also not like they're Philly and shooting 68% as a team.
The BIGGER issue, by far - they don't get to the line enough! They're getting 20 free throw attempts per game, which makes them tied for third-worst. If they were getting to the line more like 24 times/game (about league-average), that's going to have a much more pronounced effect. More free shots, but that would also mean that they're driving to the rim more and getting closer looks in general.
Great points on the percentage. I think Thomas will help us get offense into the lane a bit more, which should improve shooting percentages across the board.
I disagree about not getting to the line being a problem. I don't think that is a problem, because they are not trying to set up offenses like the Kings (who get the line the most), or the Clippers (who basically bruise you into submission with their athleticism). Their offense is more like the Blazers, who they are tied with. Last year, the Spurs had the most efficient offense in the league, but they had the fewest FTAPG per game. The Warriors, Suns, Bucks, Mavs, and Hawks all have great offenses, but they are in the bottom half of the league in FTAPG. In fact, 8 of the 15 most efficient offenses (according to Hollinger's ratings) are in the bottom half of the league in FTAPG.
10 years ago, I think FTAPG were a good way to evaluate offensive efficiency, but with the new Euroball systems, you do not need to get to the line as much. In other words, there is more than one way to skin a cat, and there is more than one way to have an efficient offense. I believe in Brad Stevens coaching ability, and I think he is implementing a great system for these players. That system will include some bruising inside with Sully (and hopefully one day Smart), but it will probably be much more like the Hawks or the Blazers system once all of the pieces get settled.
And by the way, it makes for a much more entertaining brand of basketball.