Jansen, a 22-year-old native of Curacao in the Netherlands Antilles, has been pitching professionally for less than one year, having been a catcher until the middle of last season.
The last Dodgers pitcher to reach the Major Leagues with fewer than Jansen's 56 2/3 professional innings was Yhency Brazoban, another converted position player, after 50 innings in 2004. Brazoban was 24.
"This doesn't happen all the time," general manager Ned Colletti said.
And Jansen won't be eased into action, as manager Joe Torre said he intends to use the right-hander in later innings, a decision necessitated by the continued absence of Ronald Belisario (reportedly undergoing substance abuse therapy) and the season-long struggles of George Sherrill.
"He's been a closer and I'm not saying that will happen here, but we'll use him in the later part of the game," said Torre. "Hopefully he'll blossom. You have to like his makeup."
The 6-foot-6, 220-pounder has split time this season between Class A Inland Empire -- where he had a 1.50 ERA in 11 games with 28 strikeouts in 18 innings -- and Chattanooga -- where he had a 1.67 ERA in 22 games with 50 strikeouts in 27 innings. That's 78 strikeouts in 45 innings combined with a 1.60 ERA.
"It was quick. All the stuff I did in less than a year," said Jansen, who speaks four languages. "I'm happy. I want to learn a lot more."
Jansen said he was "kind of scared" when farm director DeJon Watson suggested he become a pitcher last season "because I have a gifted arm and they wanted to see me on the mound and do a lot of special stuff." He said he didn't think he could do it until his first game.
"Ten pitches and out of the inning quick," he recalled. "I can do this. It was the right time to make me a pitcher."
Jansen credited Minor League pitching coach Charlie Hough in particular for his rapid conversion.
"Everything he told me just clicked," he said.
In addition to a fastball in the mid 90s, Jansen has an improving slider and a changeup that's still a work in progress.
The key to Jansen's rapid development has been improved control. He walked 11 in 11 2/3 innings last year, but has walked only 23 in 45 innings this year.
Prior to his conversion, Jansen was a catcher with a gun for a throwing arm, but a .229 career batting average. He played for Team Netherlands in the 2009 World Baseball Classic. He was signed by the Dodgers as a non-drafted free agent nearly six years ago.
He's not a complete unknown to the Major League coaching staff, having been added to the 40-man roster over the winter, invited to the Dodgers' winter development mini-camp and opened Spring Training in the Major League clubhouse, although he did not appear in a Major League exhibition game.
Miller had a 4.44 ERA in 19 games since his promotion May 27. The Dodgers chose risking losing Miller through waivers than optioning down rookie Travis Schlichting.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.