The guard came to Boston from Houston after a sign-and-trade deal. He said Sunday that "the expectations every year for this team is to win big and that's one thing I want to do is win big."
Lee says his friendship with Rajon Rondo played a large role in the move to Boston.
The Celtics needed to free up more money to sign Lee and wound up making a three-team deal with Portland in the mix.
Lee is set to step into the starting lineup, with Jason Terry, also signed as a free agent, coming off the bench.
The Celtics will be without the recovering Avery Bradley (shoulder) for the first two months of the season.