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What does a metalcore band do when it’s got a May-June European tour scheduled, but its vocalist is suddenly sidelined with a back injury and an inguinal hernia?

Keep calm and scream on, of course.

“We found out Alex (Varkatzas) was not going to be allowed, by doctor’s orders, to go on that tour like three or four days before we were supposed to fly,” Atreyu bassist Marc “Porter” McKnight says by phone from a tour stop in Syracuse, N.Y. “Obviously, we can’t cancel massive European festivals and stuff.”

What the Yorba Linda, Calif., band did was bring its drummer, Brandon Saller, out front and, on short notice, hire Kyle Rosa, who plays drums with Saller’s other band, Hell or High Water, as well as serving as Atreyu’s drum tech, to sit in.

It fell to McKnight to supply Varkatzas’ throat-shredding screams.

“It was fun, man. It was a different experience. But at the same time, you know, we were all thinking of Alex.”

Those good thoughts — and a couple months’ rest — were not for nothing. Varkatzas was cleared to rejoin the group for its current tour, which finds the band alongside the Used, Thrice, Circa Survive and Sum 41 as a main-stage act for the Disrupt Festival, which plays Hollywood Casino Amphitheatre on Monday.

“Yeah, Alex is back,” McKnight says. “He’s doing well. He’s in a bit of pain here and there, obviously. I mean, he just came out of surgery a few weeks ago, but he’s doing a lot better and is a lot stronger vocally.”

The band must be having a little fun with Varkatzas over his other injury, which McKnight refers to as a “linguini hernia,” before laughing and correcting himself.

As for playing festivals like Disrupt, McKnight says: “I absolutely love doing these things. There are a lot of people that are going to come out to see Circa Survive or the Used that may have heard of us but have never seen us, and now we get all these fresh faces.

“There are all these bands that are in a similar vein, even if we’re not necessarily the same genre. I think we already have made a bunch of new fans on these shows.”

Besides Varkatzas and Saller, Atreyu includes guitarists Dan Jacobs and Travis Miguel, all of whom have been in the group since the turn of the century or before. McKnight, who joined in 2004, is still “the new guy.”

The band’s latest album, “In Our Wake” (2018), is, McKnight says, “the most honest version of ourselves at this point in time. These songs just kind of poured out of us.”

Many of the songs are concerned with the future and the current generation’s legacy.

“There are songs about raising children, because (Varkatzas and Saller) are in that part of their lives,” McKnight says. “The idea of ‘In Our Wake’ came from me. I was thinking about what we as humans leave behind. Not just in the grand scheme things, but on an everyday basis. Every interaction that we have is a choice.”

Being in a band that has been around for two decades is something that none of Atreyu’s members take for granted, McKnight says.

“I’m very, very grateful that people show up to watch us play. The fact that people still care, people still love our band — that does not go unnoticed by us.

“We’re stoked. We’ve never been more stoked about our band and our career than we are right now. It’s a wonderful feeling, man.”

What Disrupt Festival • When 1 p.m. Monday • Where Hollywood Casino Amphitheatre, 14141 Riverport Drive, Maryland Heights • How much $15-$160 • More info 1-800-745-3000; livenation.com