PORT ST. LUCIE, Fla. — When the Mets acquired reliever Addison Reed last summer, he appeared to be a mere rental. But after the righthander flourished in the second half, the Mets reversed course.

Now, as spring training begins, Reed enters the season as a key setup man for closer Jeurys Familia.

“When I found out I was coming back, it was the best news ever,” said Reed, who posted a 1.17 ERA in 17 appearances after his trade from the Diamondbacks.

A return wasn’t always in the cards. Reed, 27, was eligible for arbitration and projections at the time of the trade pegged his salary at more than $5 million. That would have been steep for a typical middle reliever.

But Reed proved to be much more. Instead of non-tendering the righty, the Mets signed him to a one-year, $5.3-million deal and avoided arbitration.

“I was hoping,” Reed said of a return to the Mets. “That was the plan. I had a blast last year, not just making it to the World Series, but the group of guys here. Everything was awesome.”

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Perhaps the change of scenery did Reed some good. The trade had sent him from the languishing Diamondbacks to the Mets, who were already on their way toward knocking off the Nationals for the NL East crown.

“That last month, every game was a playoff-time atmosphere,” Reed said. “Just trying to win every game and make the playoffs. Like I said, it was a blast. It was some of the most fun I had playing baseball. Making it to the World Series was a cherry on top.”

Part of the Mets’ reasoning for bringing back Reed stemmed from his work with pitching coach Dan Warthen. He didn’t make any radical mechanical changes. Instead, Reed simply shortened his delivery, which translated into the control that had eluded him earlier in the season.

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“I felt comfortable throwing all my pitches for strikes and I think that was the biggest thing,” Reed said. “I was able to throw my slider where I wanted to throw it whereas in the first half of the season, I couldn’t tell you where the ball was going.”