OMAHA, Neb. — It was a midweek flashback for LSU Monday night.

And the Tigers were in the midweek form that frustrated so many fans in midseason.

Fortunately for the Tigers, Oregon State’s 13-1 rout only embarrassed them. It didn’t eliminate them.

“This doesn’t happen to us very frequently,” LSU coach Paul Mainieri said. “It wasn’t much fun out there for us.”

The first grand slam in TD Ameritrade Park history only put the Tigers’ out of their misery.

LSU was slowly self-destructing well before JD Harrison’s bomb in the fifth put OSU up 8-0 and made the rest of the Beavers’ romp a foregone conclusion.

When the Tigers decided to end their 17-game winning streak, they didn’t fool around.

“Nothing much went right for us,” said Mainieri said.

It began when starter Eric  Walker had to leave the game while pitching to his first batter in the third inning when his forearm tightened up, much as it did after he threw an intrasquad game last week.

“He had gone through a bunch of rehab,” Mainieri said. “Threw a couple of side sessions and felt great, felt 100 percent. His warm-ups today,  in the bullpen was great. His first two innings were great.”

LSU trailed 1-0 at the time, after the Beavers manufactured a run in the first.

“It looked like he was settling in and going to do great,” Mainieri said. “I don’t think it’s anything serious. I think it’s just a fatigued muscle. We’ll see if he’ll be available if we’re fortunate enough to get to the final round.”

Caleb Gilbert came on and gave LSU 2.2 good innings before a Robertson error opened the door for two runs and a 3-0 Beaver lead.

That started the string of eight total pitchers for the Tigers, all but one of whom contributed to the 12 walks Oregon State feasted on.

“It wasn’t what we had in mind, that’s for sure,” Mainieri said. “But we still have a breath and we’ll bounce back Wednesday, one game at a time at this point. It’s tough but it’s been done.”

That seemed to be Mainieri’s theme — after flushing away Monday’s nightmare.

It not only was LSU’s worst loss of the season, it was the Tigers’ worst loss in the College World Series since a 20-4 loss to Cal State Fullerton in 1994, some 23 years ago.

LSU (49-18) will now face an elimination game Wednesday at 6 p.m. in a rematch with Florida State, which stayed alive with a 6-4 win over Cal State Fullerton earlier Monday.

“We still have a lot to play for,” Mainieri said. “Hopefully we’ll get another shot at Oregon State later in the week.”

The top-seeded Beavers, now 56-4 and vying to get into the conversation as one of the best college teams of all time, will be off until Friday when they play the LSU-Florida State survivor.

“We probably caught (LSU) on a day where they did not play as well as they normally do,” Oregon State coach Pat Casey said. “And we played very, very well.”

LSU didn’t.

But at least there will be no finger-pointing — it was a total team effort.

It will be easy to point to the season-high 12 walks LSU allowed while forced to scrape out the bottom of the bullpen to use eight pitchers after starter Eric Walker had to leave the game in the third inning.

“Most of them had a difficult time throwing strikes,” Mainieri said.

But they had company in letting the game get away from the Tigers.

Every time we made a mistake, they took advantage of it,” Mainieri said.

And there were plenty of those.

LSU was charged with just two errors, but the Tigers had numerous defensive lapses and the bats spent the night popping up while managing only four hits — two of the ninth, well after the fact.

“We were having a tough time solving their pitcher,” Mainieri said. “We knew what to expect, but we didn’t have a very good approach against him.

“I hope we get a chance to face him again, we’ll re-evaluate our approach.”

There will be work to do before that happens.

The Tigers will use senior lefthander Jared Poché in that elimination game against the Seminoles.

They’ll have some other stuff to clean up.

Monday LSU was charged with only two errors — both by shortstop Kramer Robertson, with the first leading to two unearned runs — but the Tigers had countless misplays and mental lapses that kept the speedy Beavers in business.

Harrison’s grand slam was the defining moment during the fifth and six inning when the Beavers scored seven runs.

In those two innings the Tigers walked seven and hit another — all of whom scored.

It probably wouldn’t have mattered while the Tigers were managing just two hits off of Oregon State starter Bryce Fehmel, only scoring by the grace of Zach Watson’s solo home run in the seventh. He also had LSU’s first hit in the second inning.

Otherwise the Tigers had only one runner in scoring position — and only for one batter.

Two of their hits came in the ninth after Fehmel took a seat.

LSU pitcher Eric Walker works against Oregon State in the first inning of an NCAA College World Series baseball game in Omaha, Neb., Monday, June 19, 2017. (AP Photo/Nati Harnik)

Nati Harnik