Crowder later weighed in via Twitter.
Boston spent a total of $73 million Wednesday, including about $24 million for next season.
The Celtics had acquired Crowder in a December blockbuster that sent Rajon Rondo to Dallas.
Crowder produced career highs while averaging 9.5 points, 4.6 rebounds and 1.4 assists over 24.2 minutes per game in 57 appearances for Boston, including 17 starts.
The 6-foot-7, 240-pound Crowder showed he could play and defend multiple positions, often allowing Boston to go small with reserve lineups that featured him at power forward.
The 24-year-old Crowder averaged 10.8 points, 5 rebounds, 2 assists, and 1 steal over 25 minutes per game during Boston's first-round series against the Cleveland Cavaliers.
He was elevated to a starting role in the final game of the series.
The Celtics extended a $1.2 million qualifying offer to Crowder after the season, making him a restricted free agent.
The 34th pick in the 2012 draft, Crowder originally signed a three-year, minimum-salary deal after being acquired by the Mavericks.