The Astros granted Lambert his release Tuesday, Matt Kawahara of the Houston Chronicle reports.
The team informed Lambert on Sunday that he won't be included on the Opening Day roster, and the 28-year-old righty has now decided to explore his options in free agency. He posted a 2.92 ERA and 1.70 WHIP over 12.1 innings in the Grapefruit League but owns a 6.28 ERA across his MLB career, so he may have to settle for a minor-league contract.