Sandy Alderson has a new job, joining his protégé Billy Beane in Oakland this week as the Athletics’ senior adviser to baseball operations, but there’s a decision during his tenure as the Mets general manager that still bothers him.

“The one thing I regret — and I told him this — was not signing Daniel Murphy,’’ Alderson said by phone Friday. “We’ve kept in touch. He’s a great person and a really good player.”

At the time, the Mets were coming off their first World Series appearance in 15 years and they got there largely thanks to Murphy’s bat. Murphy finished the regular season on a tear and was named NLCS MVP.

Instead of bringing Murphy back on a long-term deal, the Mets acquired Neil Walker and a disappointed Murphy — who had hoped to stay in Queens — went to the Nationals for three years and $37.5 million.

Murphy went on to have the two best seasons of his career, finishing second in MVP voting in 2016 before he was traded to the Cubs last season. He inked a two-year, $24 million contract to join the Rockies this offseason.

“He’s a very good player and an excellent offensive player,’’ Alderson said. “We made him a qualifying offer and he signed a multi-year deal elsewhere. I don’t think we predicted him sustaining that level of performance and he didn’t end up being part of our strategy. But I applaud him for what he’s done. I’d like to have that one back.’’

Had Murphy accepted the $15.8 million qualifying offer, both sides likely would have benefitted, given how well Murphy played in 2016.

Alderson and the Mets have moved on, with Alderson, 71, having stepped down as GM in late June after his cancer returned. He was replaced following the season by Brodie Van Wagenen. Alderson announced over the weekend he was cancer-free and jokingly said he was waiting for free agents Manny Machado and Bryce Harper to sign before taking a new job.

Alderson, who initially resurfaced at the winter meetings in December, sad he did so for a number of reasons.

In addition to being on MLB’s rules committee, which was meeting in Las Vegas, Alderson also wanted to reassure the baseball world that he was OK.

“I went, in part, not because I was looking for a job, but to make sure people there knew I wasn’t down and out,’’ Alderson said.

His talks with Beane began informally in November, not long after Alderson officially left the Mets.

While he wasn’t necessarily searching for a full-time job, he said he needed “an outlet for my energy and opinions.’’

Alderson said he intends to commute from New York for the time being and will keep an eye on his former players — the younger ones in particular.

“There are guys that are close, like Peter Alonso, that I want to follow,” Alderson said. “And [Jeff] McNeil was a revelation last year. And [Brandon] Nimmo, [Amed] Rosario and [Michael] Conforto could be big contributors. I’ll certainly follow the Mets.”

First-year Mets general manager Brodie Van Wagenen announced Friday on Twitter he will sit with the 7 Line Army for a game July 2 against the Yankees.