Namestnikov had a career-high 48 points last season. He had 20 goals and 24 assists with Tampa before adding two goals and two assists in 19 games with New York.
The 25-year-old was coming off a two-year $3.875 million contract.
Namestnikov came to New York with Brett Howden, Libor Hajek and draft picks for McDonagh and J.T. Miller.