NEW YORK -- Sean Manaea, one of baseball's top pitchers in the second half of the season, opted out of the $13.5 million option on his contract and became a free agent, the Mets announced Monday.

The decision comes as little surprise given Manaea's accomplishments during a season that saw him go 12-6 with a 3.47 ERA over 32 starts, demonstrating notable improvement after dropping his arm angle in July. The 32-year-old is likely to seek and receive a multi-year deal on the open market.

But first, Manaea must get there. His opt-out decision will likely prompt the Mets to extend him a qualifying offer, which is a one-year contract worth $21.05 million. If Manaea rejects that, which is likely, the Mets would receive Draft pick compensation in the event he signs elsewhere.

At the conclusion of the postseason, Manaea expressed interest in a return, saying: "I love my time here. I love New York. I love the organization. I love all the people here."

It remains to be seen if the Mets will be among the clubs willing to commit to Manaea on a multi-year deal, or if they will look elsewhere in free agency. The left-hander's opt-out decision does not in any way preclude a return to New York; it simply gives him and his agent, Scott Boras, a chance to negotiate with all 30 teams.

The Mets are seeking multiple starting pitchers on the open market this offseason, with Manaea, Luis Severino and Jose Quintana all hitting free agency. Those pitchers combined for 37 percent of the team's innings this season.

Manaea is a veteran of nine Major League seasons, spending the first six of those with the A's before bouncing to the Padres, Giants and Mets. He joined New York last offseason on a two-year, $28 million deal that included his $13.5 million contract option.