Sandy Alderson revealed Tuesday that he underwent a second surgery earlier this month to combat the undisclosed cancer he’s been battling since the end of last season.

Alderson, 68, called a news conference a few hours before the first pitch of Tuesday night’s showdown with the Nationals at Citi Field to announce the latest round of his cancer treatment, explaining that he wanted to head off speculation about the operation, which was May 11.

“The surgery was pre-planned,” Alderson said. “It was part of an overall strategy that was devised by my doctors early in December. The surgery followed a very successful regime of chemotherapy, and the surgery itself was, um, what can I say, fantastically successful.

“The doctors were able to do everything they had hoped to do and were able to avoid everything they had hoped to avoid. So from that standpoint, everything worked out well.”

Alderson has been quietly dealing with the cancer since last September, when he was first given the diagnosis shortly after the Mets clinched the National League East crown. But the GM kept it under wraps until the initial surgery in December, which prevented him from attending MLB’s winter meetings. Otherwise, Alderson has gone about his business, with almost no concession to the illness.

“We’ve all known someone who’s had to fight a fight,” Terry Collins said. “I’ve never been around anybody who’s fought it better. This guy absolutely never lets his condition stand in the way of anything he does, and the way he continues to work with me and the team. We know he’s not feeling well, but that’s never been in the way. I’ve never been around somebody stronger, that’s ever been through something like this before, ever.

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“The fact that he’s here today after he just had surgery a few days ago tells you how tough he is. It was great to see him and we’re glad he’s doing well.”

Alderson joked about having a dry mouth as he recovers from the procedure, adding that he would be on somewhat of an abbreviated work schedule until he’s at full strength again.

“I’m not running a four-minute mile at the moment,” Alderson said. “My mouth is a little dry from time to time. But that’s what happens when you have surgery of any type.”

The GM said he did not expect to need another surgical procedure, but his doctors were unsure about additional chemotherapy. The GM still felt it important, however, to keep the nature of his cancer private.

“It’s not because I’m trying to be cute or evasive,” Alderson said. “But there’s a certain amount of information that is mine only, and rather than throwing everything out there and creating even more speculation . . . I’ve just decided not to go there.”