SEATTLE — The “crash course” Joe Girardi said Chase Headley would be getting at first base began in earnest Thursday.

Not only was the 33-year-old on the field mid-afternoon at Safeco Field working at the position with infield coach Joe Espada, Headley was in the lineup there — against Felix Hernandez no less — for that night’s game against the Mariners.

Nothing like being thrown right into the fire.

“I think he’s going to do just fine,” Joe Girardi said late Thursday afternoon. “There’s going to be an adjustment period and there might be some times where he finds himself in the wrong spot, but I think he has a pretty good idea of what he needs to do. His work looked good today. The thing is he’s seen it a little bit in spring training and he’s seen it in a few games from times to times. But there still are a lot of situations he hasn’t seen and we’ve got to live with it.”

The position change became necessary when the Yankees acquired Todd Frazier, as well as relievers David Robertson and Tommy Kahnle, from the White Sox

Frazier, 31, actually has more experience in his career at first but has primarily been a third baseman in his career and, frankly, has the better career pedigree.

Headley, slashing .257/.339/.368 with four homers and 37 RBIs entering Thursday night’s game, has started six games in his career at first base, compared to 1,076 at third. Frazier, slashing .206/.329/.431 but with 16 homers and 44 RBIs, has started 720 games at third, counting Thursday night, and 82 games at first.

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“It’s still the infield, it’s still doing the same types of things, it’s just a little bit different angles and responsibilities, so I’ll do the best I can to pick it up as quickly as I possibly can,” Headley said. “I don’t anticipate it taking too long.”

As Girardi put it: “The only thing that’s going to help him is experience.”

Headley indicated on Wednesday, shortly after he received the news from Girardi that he would be switching positions, his preference was to play there right away.

“I’d rather play there every day and get more consistent time over there than jumping back and forth [between third and first] so I think that will make it a little bit easier. But obviously in the middle of the season is a little bit different than picking it up in the offseason, so I’ll just do the best I can with it.”

Headley also acknowledged the challenges involved.

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“It’s kind of like with the shift, you’re just seeing different spin and different angles,” he said. “A lot of times balls are going the opposite way they’re spinning away from you. Just learning that and also to make sure I’m thinking through all the responsibilities that are a little bit different — the cutoffs, especially double-cuts, things like that are a little bit different . . . bunt coverages, those types of things. But I don’t anticipate having too much trouble with it.”

Girardi said he would consider using rookie Garrett Cooper, acquired last week from the Brewers, as a late-inning defensive replacement. Cooper, a righty bat who has three doubles in six games, also figures to start against lefty starters as part of a platoon with Headley.

“I’m glad that our team’s better,” Headley said. “Do I think I can still play third? Of course I do, but if this makes our team better I’m happy to do it. That’s what I want. I want to go to the playoffs, I want to win a World Series, and if they think this gives us a better chance, then I’m in for it.”