Minnesota State Fairgoers fumed. On a few hours' notice, they learned last September that the Doobie Brothers would perform at the grandstand without Michael McDonald, who had just rejoined the group after a 26-year absence.

Just think how McDonald felt, sidelined with his second bout of COVID-19.

The Rock & Roll Hall of Fame band arrived in the Twin Cities the day before their gig.

"The night I came in is when I started to get pretty sick," said McDonald, who returns with the Doobies on Friday at Treasure Island Resort & Casino amphitheater. "We had a certain protocol we had to follow. Anybody who gets COVID can't be in the building, can't be backstage."

Cutting a few songs from their setlist, including "What a Fool Believes," the Doobies did the State Fair show without McDonald because there was no flexibility to reschedule. Several ensuing concerts were postponed and rescheduled.

McDonald quarantined at a Minneapolis hotel. He didn't remember the name of the place, but he remembered the room.

"My TV didn't even work and I didn't even care. That's how sick I was. I was just lying in a room with the sound on just to hear other voices. They wouldn't even give me room service. I had to order in food from outside."

McDonald got "pretty sick. Mostly fatigue," he recalled.

"I'd been vaccinated, so it didn't get into my lungs. I was grateful for that. I was so bedridden otherwise. ... Getting up and going to the bathroom took everything."

After quarantining, he received antibodies and felt better, though he still has lingering symptoms.

"I have a little bit of COVID brain. I feel like there's a fogginess I can't seem to shake. Things I know damn well shouldn't be that hard for me are. Playing all of a sudden, I realize my dexterity is not what it should be."

This wasn't the 70-year-old's first bout with COVID. He feels that he experienced it in September 2019 — six months before the pandemic — while on a solo tour with Chaka Khan.

"I got really sick. Sicker than I've been before. I discovered that I didn't have my sense of smell but none of that stuff was being talked about yet. COVID wasn't even being talked about."

Doobies as bossa nova tunes

The Doobies' 50th anniversary tour started in 2019 and, thanks to the pandemic interruption, might go on longer than Elton John's farewell tour.

"I don't know how long we intended to go," McDonald said. "I hope it keeps going. It's been wonderful."

He joined the Doobies in 1975 as a temporary replacement for injured singer-guitarist Tom Johnston but McDonald became a full-fledged member, singing such hits as "Takin' It to the Streets" and "Minute by Minute." He left in 1982 to pursue a solo career ("I Keep Forgettin'," "On My Own" with Patti LaBelle) and rejoined briefly in 1987, '92 and for a '95-'96 tour.

Returning for the anniversary tour was like getting back on a bicycle, he said.

"It's always been an easy hang for me with the guys. We've always been friends. Even the years when I wasn't playing with them, we always stayed in touch. A lot of it being we all had kids the same age and the kids knew each other. I'd do special events with them from time to time."

The keyboardist-singer did not join the Doobies in the studio for last year's "Liberte" — their first record in seven years — because he was touring solo when they recorded pre-pandemic.

During the pandemic, McDonald recorded a dozen solo singles that he has been releasing one at a time. They are bossa nova re-imaginings of Doobies songs arranged by Tony Ybarra, a Santa Barbara flamenco/classical/jazz guitarist who has been giving McDonald guitar lessons. The latest single is "You Belong to Me," a duet with Spanish singer Buika.

McDonald is uncertain what's next. Maybe an overseas Doobies tour in 2023. Maybe a one-man show like he presented at New York's Cafe Carlyle. Maybe gigs with orchestras.

Definitely on his schedule is the Smooth Jazz Cruise in January and February. McDonald was recruited as one of the singers accompanied by an all-star house band led by bassist Marcus Miller. The keyboardist is not embarrassed by the oft-derided smooth-jazz label.

"I think some of the best players I know are in that genre. It's funny. It's largely easy listening, a lot of it. You'll hear them play stuff that's palatable for smooth-jazz radio but they'll play other stuff that's much more cutting-edge and much more progressive that doesn't really have a radio format that embraces it."

Wait! Smooth jazz? Isn't McDonald an essential exponent of yacht rock?

Despite the yacht-rock stigma, he's not complaining. When the '80s were ending, the Doobies and other soft-rockers were embraced by oldies radio.

"You've got to be grateful for that," he said. "It allowed us to keep working and go out on the road and play for our audiences."

Last year, Christopher Cross told the Star Tribune that the ultimate yacht-rock tour would feature himself, McDonald, Toto, Hall & Oates and Steely Dan.

"Ha-ha," McDonald said. "No short of neurosis there. We all come from the generation of [eliminating] yellow M&Ms [backstage]."

There is no more definitive yacht-rock song than "What a Fool Believes," the Grammy-winning Doobies tune McDonald wrote with Kenny Loggins.

"I always felt the song was about how two people, who are intimately involved with each other, can see the same thing totally differently. How they totally not communicate," McDonald explained. "It gives this guy a chance to reunite with the love of his life and it was an epic love affair. To her, he was more or less a good friend and maybe they had a little fling. It was never more than that for her.

"And even getting back together for dinner, they walked away with two totally different ideas of the same moment. He thought the door was opening for the chance he felt he always missed and to her, it was just reminiscing with an old friend."

Any interview with a Doobie Brother requires the mandatory question: What's the best Doobies song with which to smoke a doobie ('60s slang for a joint)?

"'Clear As the Driven Snow.' It's kind of a progressive rock with a lot of guitar work. It's a song you'd imagine someone smoked a lot of pot before they wrote it or dropped some acid. It has a lot of movements. If I still smoked pot, that would be a song I'd enjoy listening to."

Doobie Brothers

When: 8 p.m. Fri.

Where: Treasure Island Resort & Casino amphitheater, Red Wing.

Tickets: $39-$129; ticketmaster.com.