Thomas will miss Friday's game against the cellar-dwelling Brooklyn Nets (12-54) and Sunday's matinee with the Philadelphia 76ers (24-43).
Boston (43-25), which hosts Washington (41-26) on Monday, holds a 1 1/2-game lead over the Wizards for the No. 2 seed in the Eastern Conference.
Thomas scored 27 points and logged 32 minutes in the Celtics' 117-104 victory over the Minnesota Timberwolves on Wednesday.
The 28-year-old is averaging a career-best 29.2 points per game this season for Boston, which had a 1-3 record in the four games he missed in December with a strained right groin.