"Yes, he's out," Doc Rivers told the Boston Globe about Kevin Garnett's status. "I feel for the season, but we just don't know yet."
Rivers initially made the surprise announcement during a radio interview earlier on Thursday.
The injury keeping Garnett on the sidelines is a strained popliteus tendon, not the bone spur which Rivers said will likely require off-season surgery.
Garnett's knee is only 70 percent healed.