SEATTLE –– Some 2 ½ hours before game time yesterday afternoon, bench coach Brad Mills took down the original lineup card he had tacked up to the bulletin board and replaced it with a revised one.

The first one had featured the standard Red Sox batting order, including Kevin Youkilis playing first base and hitting sixth; the second one was without Youkilis, who was battling a sore right hand, and instead had Sean Casey at first, hitting eighth.

For Casey, the news was a mixed blessing. It offered him a spot in the starting lineup for the first time since May 17. But it came on a night when the Sox were facing Felix Hernandez of the Seattle Mariners, whose stuff can be as good as any right-hander in the league.

Then again, that’s the role that Casey signed on for when he agreed to a one-year deal with the Red Sox in the first week of February.

Until this year, Casey had only once (in 2006) had a season in which he had fewer than 400 at-bats, and that came when he missed most of the first two months of the season with injuries.

Even last year, when Casey lost his job as the Detroit Tigers everyday first baseman to Carlos Guillen in second half of the season, he still managed to accumulate 453 at-bats.

Coming to the Sox, Casey knew his playing opportunities would be limited. With Youkilis established at first and David Ortiz, the game’s premier DH, he knew he would have to wait for opportunities.

The wonder is that he has done it so well, so quickly. Heading into last night’s showdown with Hernandez in the first game of the series here, Casey was hitting a team-high .355 in 62 at-bats, covering 20 games. At the current pace, Casey won’t finish with 200 at-bats, but clearly, he’s making the most of his playing time.

“You never get used to (not playing),” said Casey yesterday. “I’ve just been trying to make sure I stay in my routine and that I get my work in so that when I’m called upon, I’m ready.”

Casey knew the transition would be difficult. A three-time All-Star who came into this season with a .301 career batting average, he was accustomed to coming to the ballpark and assuming he would be in the lineup.

Then there’s his personality. Engaging and sociable, Casey is one of baseball’s most popular and friendliest personalities and the downtime in the dugout has caused another adjustment.

A self-described “Type A” personality, Casey has had find outlets for his energy other than the batter’s box or the first-base bag. To help him cope, he’s solicited the advice of hitting coach Dave Magadan, who in the second half of his playing career, became more of a role player.

“I think it helped that he started to go through that transition last year,” said Terry Francona, who, while making clear the limits of his role, personally called Casey in the offseason and recruited him for the Sox. “But it’s still a hard thing to handle. I think it’s a little easier for veteran guys like Case.”

Beyond the mental adjustment, there’s the matter of succeeding when you get the chance to play.

Francona said Casey’s swing is well-suited for the role, since it’s compact, making him less prone to mechanical hitches that can be difficult to remedy in the batting cage alone.

Also, Casey’s ability to hit the ball the other way makes him perfect for Fenway Park.

Had Casey signed with a National League team, he knows, he would be more involved. Thanks to double-switches and pinch-hitting for starting pitchers, role players see more regular playing time.

“Over there,” said Casey, a career National-Leaguer until last season, “you’ve got a good shot at getting in almost every night. Over here, it’s not like that, especially with our lineup. So you just have to stay ready.”

It helped that when Mike Lowell missed almost three weeks with a sprained thumb, Casey got the chance to play almost every day in April. He made the most of it, too, before incurring his own injury — a hip pointer — that forced him to the DL.

Now, with Lowell healthy and Youkilis back at first, Casey bides his time and tries to stay focused and available.

“The kind of hitter I am,” he said, “I always felt that when I get into the box, I’m ready to get a hit. I always want to be prepared, so that when it comes time to play, there are no excuses.”

Even on short notice, such as last night, against the likes of Felix Hernandez.

Next Game

Tonight at Seattle

10:10 p.m.Red Sox at Seattle

For a complete report of the game, go to: projosports.com/redsox

smcadam@projo.com