Actually, he was 13-24. One of the things that impresses me about him is that he doesn't get his 30 by chucking up 30+ shots - he's actually a very efficient offensive player.
Part of why he is scoring with such ease of late is because his teammates are hitting their 3s - right now they are on a run of 4-5 straight games of hitting at least 17 threes, not all of which are Isaiah's.
Contrast this to last season's playoffs. During the regular season last year, Boston's leading three point shooters were Thomas (167 made threes at 36%), Bradley (147 at 36%), Crowder (122 at 34%), Olynyk (85 at 41%), Smart (61 at an awful 25%), and Jerebko (43 at 40%).
But in the playoffs, Isaiah did not have this kind of support. He didn't shoot well from 3 (13 makes at 28%), Bradley went 1-7 in game 1 before going out for the playoffs, Crowder was caming back from the ankle injury and didn't shoot well (only 10-41), Olynyk was on his was to shoulder surgery and was ineffective (0-3 from deep), Turner was lousy as expected (3-14). The only guys who helped even a little were Smart (11 makes at 34%), Jerebko (7 at 32%), Rozier (4 at 36%).
Even last year, if you have a healthy Bradley, Olynyk, and Crowder, it's a very different playoff run.
And Isaiah's main problem isn't that he's 5'8", it is that he's the only scorer on the team. Any other guy would have the same problem.