Difficulty level is about to rise for Angels starter Jerome Williams

MINNEAPOLIS — This is not, as ballplayers like to say, Jerome Williams’ first rodeo. The 30-year-old right-hander is in his 15th professional season, has spent nearly four years in the big leagues, started a playoff game and has pitched in Mexico, Venezuela and Taiwan.

But even Williams had to admit that it will be a considerable jump going from a rehabilitation start for Class-A Inland Empire on Tuesday night to pitching Sunday night against the New York Yankees in Yankee Stadium in a game that will be nationally televised.

“I’m going from facing kids in high-A ball to facing almost a whole lineup of Hall of Famers in Yankee Stadium,” said Williams, who joined the Angels in Minneapolis on Wednesday after recovering from a left hamstring strain. “It’s kind of nerve-racking, but I have to look at it as just another ballgame.”

In a bit of serendipity, Williams, who is African American, will wear No. 42 on Sunday as part of Jackie Robinson Day, which baseball commemorates every April 15. Before joining the Angels in August, Williams hadn’t pitched in the big leagues since 2007. It wasn’t until 2008 that all players began wearing No. 42 on April 15.

“I always wanted to wear No. 42,” said Williams, who didn’t realize Sunday was Jackie Robinson Day until reporters informed him. “That’s awesome. And I’ve never pitched in Yankee Stadium. Wow, that’s a lot of firsts.”

Williams showed he was sound by giving up two runs and six hits, striking out four, walking none and covering first base three times in six innings against Lake Elsinore on Tuesday night.

He earned first crack at the fifth rotation spot over prospect Garrett Richards by going 4-0 with a 3.68 earned-run average in 10 games after his Aug. 17 promotion last season.

“Trusting my stuff was the key,” Williams said. “And if something bad happened before, an error or a hit, I’d drag it with me the whole game. I realized I have to pick guys up sometimes, and I can’t let things like that snowball.”

Michael Kohn out for season

What was originally diagnosed as a forearm strain turned into a major elbow injury for reliever Michael Kohn, who will undergo ligament replacement (Tommy John) surgery Thursday in Florida and will be sidelined for 12 to 18 months.

Kohn threw five scoreless innings in spring training before being shut down in late March. After suffering another setback when he tried to throw long toss last week, the right-hander traveled to Florida to be examined by James Andrews, the surgeon who will perform Thursday’s procedure.

Kohn was not expected to play a prominent role for the Angels this season, but the injury will put a dent in the organization’s bullpen depth.

Short hops

Alberto Callaspo started at third base Wednesday night, in part because of his .364 average (12 for 33) against Minnesota Twins right-hander Carl Pavano, but Manager Mike Scioscia said Mark Trumbo would start at third in Thursday’s series finale.

mike.digiovanna@latimes.com