It’s the good ol’ Grateful Dead — well, three core members — with another new singer/guitarist. Ready for this?

It’s John Mayer.

Yes, that John Mayer, the pop star with the big mouth, big blues guitar licks and big list of former famous girlfriends, including Jennifer Aniston, Taylor Swift and Katy Perry.

Deadheads sighed. Cynics whined. People were skeptical.

“That’s for sure, me included,” Dead drummer Mickey Hart admitted this week. “I never thought it could happen. He’s a blues player, he plays the hits, and he sings.”

But halfway into a 21-concert tour, Hart sounds super-excited about this new band, dubbed Dead & Company, that’s coming to Target Center on Saturday.

Mayer is “a very schooled musician,” Hart observed. “He went to Berklee [School of Music]. He’s fluid. He can play in the Grateful Dead language. He’s fast and clean and lyrical. He can go anywhere. Like Jerry. He’s not a Jerry. He’s not trying to be Jerry.”

But Mayer, 38, went to school on the Grateful Dead, the Rock and Roll Hall of Fame band known for its trippy jams led by singer/guitarist Jerry Garcia, who died in 1995, and devoted, tie-dyed Deadhead fans. Mayer had to learn the entire Dead songbook.

“I don’t know how he did it,” Hart said. “I’m more than impressed. He took it seriously.”

The new lineup practiced for more than a month — eight or nine hours a day, according to the band’s other drummer, Bill Kreutzmann.

“We do 100 songs,” he said. “All Grateful Dead songs. There was no time to get to [rehearse] covers.”

Both drummers agree, Dead & Company is a new endeavor.

“For a new band, it’s over-the-top good,” Kreutzmann said of the tour thus far.

Hart, as he is wont to do, put it in more flowery hippie-meets-new-age language: “It’s always great to witness an organic group birthing itself.”

Right on.

Don’t confuse Dead & Company with this summer’s ballyhooed “Fare Thee Well” stadium shows in Chicago and Santa Clara, Calif. Those five celebrated marathons to honor the Grateful Dead’s 50th anniversary featured the so-called “core four” of the surviving Dead — Hart, Kreutzmann, singer/guitarist Bob Weir and bassist Phil Lesh — plus singer/guitarist Trey Anastasio of Phish and pianist Bruce Hornsby, among others.

Lesh isn’t on board for Dead & Company.

“Phil doesn’t want to tour. It’s as simple as that,” Hart said curtly.

Well, it’s probably not as simple as that because Lesh, 75, who has bladder cancer, performed several East Coast concerts in theaters in September and October with his side project, Phil Lesh & Friends.

The bassist for Dead & Company is Oteil Burbridge, best known for a long stint in the Allman Brothers. On keyboards is Jeff Chimenti.

Mayer is the new member who’s hard to figure out. Hart and Weir met him when they were visiting with producer-turned-label-president Don Was at Capitol Records in Hollywood. Mayer was recording in the same building and Was introduced them. Mayer, who was “bit by the Grateful Dead virus,” as Hart put it, invited Weir to perform with him on “The Late, Late Show” in February. And the relationship evolved from there.

Future for new band

Do the Dead drummers feel guilty about hitting the road so soon after Deadheads dropped big bucks to attend those alleged farewell shows?

“Nobody’s getting ripped off,” Hart said of the Fare Thee Well tickets that cost $59 to $200 plus fees. “That’s what it was. This was us coming together for the 50th anniversary to say thank you to them and they said thank you to us big time. We didn’t have any idea what we’re doing now was going to happen. Enjoy this while you can.”

Dead & Company appears to have a future. “Absolutely,” Hart said with enthusiasm. “You couldn’t walk away from something like this.”

Is there a future for the Fare Thee Well lineup?

“We haven’t spoken per se about that. But if it were to come around next year and we talked it out and made it as smooth as possible, I would do it again,” Kreutzmann said. “I never say no to good things Grateful Dead.”

There appeared to be some sourness during the Fare Thee Will shows. On one night in Chicago, Weir came out for the encore wearing a T-shirt emblazoned with “Let Trey Sing.”

“There were no hard feelings,” Kreutzmann noted.

How does Anastasio compare to Mayer?

“Apples and oranges,” Kreutzmann said.

Said Hart: “They are completely different. Trey is a marvelous player. I only played with him a few times. I know John’s playing more intimately.”

Stage is church

Each night for Dead & Company, Weir and Mayer plot out a set list, according to Hart. No set lists have been the same.

How do they choose who sings what?

“That’s a good question,” Kreutzmann said.

Despite being unrehearsed, some covers have crept into the sets, including “Midnight Hour” (“everyone knows ‘Midnight Hour,’ ” Kreutzmann says) and “Get Out of My Life, Woman,” a tribute to the late Allen Toussaint.

“Drums > Space,” the Dead’s classic improvisational jam, is in every set.

“It’s the most played song in the Grateful Dead canon,” Hart said. “It’s not rehearsed. It’s in the moment. Bill and I have 50 years of conversation, which is ongoing.”

At 72, Hart shows no sign of letting up.

“I’m motivated by desperation,” he said. “Trying to find that groove, the rhythm of life. This [performing] is a big part of life. That’s my church.”

Is the Mayer/Dead marriage a better move for the pop star or the band?

“It’s a win-win,” offered Kreutzmann, 69.

Considering all the post-Garcia lineups and bands — Furthur, the Other Ones, Ratdog, Lesh & Friends — adding another one to the list says something about the enduring nature of the group’s songs.

“First of all, there were amazing images. It’s [lyricist] Bob Hunter who created the whole mythology and great stories,” Hart said. “But it’s something deeper than just the songs. There’s a fifth element at work here. We don’t know what it is. But we know how to do it.”

Or he put it more succinctly: “The Grateful Dead is Santa Claus.”

Santa may come by only once a year but who knew that 2015 — with the Fare Thee Well shows and Dead & Company tour plus new books by David Gans, David Browne and Kreutzmann — would be the year of the living Dead?

“I wouldn’t have thunk it,” Kreutzmann said. “We’re doing these big shows again. I thought it would never happen again.”