Allen missed his seventh straight game with an ankle injury and Rondo has been out for two games in a row with a sore back. Neither made the trip to Atlanta. Pierce and Garnett were not in uniform, resting for the playoffs.
Even with Boston shorthanded, the Hawks had a hard time putting away the Celtics and would not have done so without the 30 points and six assists from Johnson. Josh Smith added 19 points and 12 rebounds in his 27th double-double of the season.
Atlanta and Boston are likely headed to a first-round playoff meeting. The Celtics would have home-court advantage in such a series, even though they are 1 1/2 games behind the Hawks in the standings.
Boston's division title ensures then no worse than the No. 4 seed.
Avery Bradley scored a career-high 28 points to lead the Celtics.