Allen played a pivotal defensive role for the Eastern Conference champion Celtics during the 2009–10 season, averaging 6.1 points, 2.7 rebounds, 1.3 assists, and 1.07 steals in 16.5 minutes in 77 games (eight starts). He recorded his first career double-double with 10 points and 10 rebounds on December 27, 2009 against the Los Angeles Clippers. During the 2010 playoffs, Allen played in all 24 games and averaged 5.1 points and 1.04 steals in 16.3 minutes. The Celtics reached the 2010 NBA Finals, where they were defeated 4–3 by the Lakers.