This is why there will be a lot more "parity" in the NBA this season as far as regular season record is concerned. A lot more teams will be finishing with the equivalent of 30-45 wins.
The Celtics will be one of the many teams that will have a much worse winning percentage than they might have had in regular-length season.
Come playoff time, there will probably be a number of "upsets" based on record simply because some teams will greatly overachieve in the condensed regular season (e.g. Nuggets, Magic, Hawks) and others will greatly underachieve (e.g. Mavs, Celtics).