The dream matchup between the Los Angeles Lakers and Boston Celtics as expected seems to be producing the best NBA ratings in some time. While we'll only see the primetime portion of the game (9p-11p) initially and it could be that peak viewing was in the fourth quarter as it was a fairly close game most of the way. As it is the game which Boston won to go up one game to none averaged a 9.2/15 (household/share) for the primetime (9p-11p) portion of the game in the most preliminary numbers available.
Lump in the portion of the game that occured after primetime, which may well have been the most watched portion, and it seems like it will definitely be over 15 million viewers (but well under 20 million).
Just based on the 9.2/15 during primetime it would be somewhere around 15 million and the prelim numbers for sports are often low and revised upwards. Either way most likely the best results in a few years. We'll have the viewer portion of primetime later, and as ABC will no doubt crow with a press release, we'll probably see more data on top of that later in the day.
Update: the viewer numbers are in but the preliminary HH Ratings above were...wrong. For the 9pm-10pm hour the game had a 6.6/11 and from 10pm-11p a 6.9/12. Well under a 9.2 HH Rating. The perils of reporting numbers as they come in. The viewership was significantly under 15 million. Because live events seem to get kind of screwed up for the timezones, it's hard to say. What we know is that from 8pm-11pm including a Jimmy Kimmel special and the pregame ABC averaged 9.49 million viewers and based on half hourly data between 9p-11p viewing ranged from 9.2 million to 10.35 million for the primetime portion of the game.
Live numbers for sporting events are almost always adjusted up, and I'm sure these will be adjusted up too. Again, the final portion of the game after 11pm on the east coast was likely the most-watched portion. Still, it appears 15 million doesn't seem likely even with the non-primetime portion factored in.