Grabovski, 30, inked a one-year contract with the Capitals last August. He was limited to just 58 games this season due to problems with his left ankle.
The Belarusian center registered 13 goals and 22 assists for 35 points with Washington.
Penner, 31, was acquired from the Anaheim Ducks on Mar. 4.
Last season, he picked up one goal and two assists in 18 games with the Capitals, and tallied 13 goals and 19 assists for 32 points in 49 games with the Ducks.
While the final decision on both Grabovski and Penner will presumably be made by the Capitals' next GM, it's "unlikely" both will be back next season, according to a source close to the organization.
The Capitals plan on tinkering with their roster, once a new GM is at the helm.
With roughly $15 million in available salary cap space, if the cap hits the projected $71.1 million -- though it could range between $68 million and $70 million -- the team will have to be somewhat creative in how it approaches its off-season moves.