Frank Thomas made the transition from playing in the field to serving as a designated hitter as well as anyone has ever done it.

The future Hall of Fame slugger played 90 or more games at first base in each of his first seven seasons, a span of time during which he won back-to-back MVP awards. By the time he was 30, however, he was a full-time designated hitter — and he stayed that way for the rest of his career.

But just because Thomas made it look easy — in 2000 he hit 43 home runs and finished second in MVP voting at age 32, despite playing just a handful of games in the field — doesn’t mean it’s actually easy.

“People really don’t respect the position because they’ve never been in those shoes,” Thomas said ‘last week. “But you ask any DH in the league how tough it is to sit there and pinch-hit four times a day and put up monster numbers. It’s tough.”

The difficulty of designated hitting complicates the decision the Red Sox have to make this winter with David Ortiz — arguably one of the top two or three designated hitters in the history of the game.

Ortiz will hit the free-agent market in November on the heels of his second straight terrific season, all but erasing any memories of his injury-plagued 2008 and slump-marred 2009. He has hit 20 home runs and 28 doubles in four-plus months, and he’s gotten on base at a .376 clip — which, if the season ended today, would be his best mark since 2007.

If Ortiz, who will turn 36 in November, is offered more money elsewhere than the Red Sox are willing to offer, Theo Epstein will have to find another designated hitter to replace him. It’s not as easy a job as it might seem.

A comprehensive study detailed in “The Book: Playing the Percentages in Baseball” (written by statisticians Tom Tango, Mitchel Lichtman and Andrew Dolphin) determined that hitters perform measurably better when they play the field than when they sit on the bench between at-bats.

Thomas’ old team, the White Sox, has had its own troubles at the designated-hitter position this season.

Adam Dunn spent the first 10 seasons of his career in the National League, playing first base and left field at a below-average level but hitting the ball like few else could. Dunn hit 40 or home runs in five straight seasons and 38 home runs in back-to-back seasons after that run ended, and he hit the free-agent market last winter having compiled a .381 on-base percentage in his previous seven seasons.

He looked like a perfect candidate to make the same transition Thomas made in the late 1990s.

But Dunn has been a disaster as a DH. He is hitting just .165 with a .296 on-base percentage and a league-worst 138 strikeouts. He has hit just 10 home runs all season. He managed just two hits and two walks in 12 plate appearances against the Red Sox last weekend, and every out elicited boos from the Chicago crowd.

Players talk often about separating different parts of the game from one another, but Dunn can’t do that. All the White Sox are asking him to do is hit, and he’s not hitting.

“We’ve talked about Adam Dunn all year long, and he’s been out of position all year long,” Thomas said. “I’m sure it’s a difficult adjustment for him. He’s had too much time to think about hitting. I know, when you’re in a slump like that and you sit over there as a DH, it’s really rough on you.”

Red Sox third baseman Kevin Youkilis looks like a candidate to transition to the designated-hitter role at some point in the future, and hot-hitting catching prospect Ryan Lavarnway likewise could contend to fill the role next season if Ortiz does not return.

But it’s not as easy as just sticking a good hitter into the DH spot. Lavarnway, for example, has a slugging percentage 75 points lower as a designated hitter than as a catcher since his promotion to Class AAA — and the difference was more than 150 points in his two-plus months at Class AA before that.

Part of why Ortiz has been so valuable for so long — and why Thomas is going to go to the Hall of Fame — is because he thrives in a role in which not every hitter is comfortable. If Ortiz gets offered big money elsewhere this winter, the job of replacing him won’t be as simple as one might think.