Ervin Santana scratched from Tuesday’s game

The Angels don’t think Ervin Santana’s latest elbow setback, which forced the right-hander to be scratched from tonight’s start against the San Francisco Giants, is serious, but it is definitely cause for concern.

Santana sat out the first six weeks of the season because of an elbow ligament sprain and has not come close to regaining his 2008 All-Star form, going 1-3 with a 7.47 earned-run average in six starts.

Then he began feeling tightness on the outside of his forearm, just below the elbow, while throwing his fastball in Thursday’s game at Tampa Bay.

Santana, who gave up six runs in 4 2/3 innings of an 11-1 loss to the Rays, underwent an MRI test Monday, the results of which were not yet available.

Right-hander Sean O’Sullivan, who is 5-2 with a 6.02 ERA at triple-A Salt Lake, will be called up to make his major league debut tonight. To clear roster room for O’Sullivan, the Angels will put pitcher Kelvim Escobar on the 15-day disabled list.

“I felt pain, but it’s not in the same area” as the ligament sprain, Santana said. “That was on the inside of the elbow. This is on the outside. I know I’m going to be OK. I think the nerve is just a little fatigued.”

With a day off Thursday, the Angels won’t need a fifth starter until June 23. Manager Mike Scioscia hopes Santana, who gave up one run and six hits in 8 2/3 innings of his previous start, in Detroit on June 5, is ready to pitch that night against Colorado.

“If I hadn’t seen him throw as well as he did in Detroit and hadn’t seen his stuff pick up like it has, I wouldn’t be as comfortable with where he is,” Scioscia said. “But we’re going to pay close attention to it, that’s for sure.”

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It’s a setup

Escobar, whose surgically repaired shoulder did not respond well to his June 6 return to the rotation, didn’t view his disabled list stint as a setback. The right-hander will just have a little extra time to prepare for his move to the bullpen.

Scioscia said he’d be “very surprised if he was not in our bullpen next Monday,” the day Escobar is eligible to be activated.

“They needed to make a move because they know I’m not going to be ready in three or four days,” said Escobar, who began playing catch Monday. “There’s not a switch you can turn on and off.”

With Scot Shields out for the year because of knee surgery and Jose Arredondo demoted to Salt Lake, the Angels hope Escobar can seize an eighth-inning role as he did in 2005, when he returned from elbow surgery to go 1-0 with a 1.89 ERA in nine September relief appearances.

“I don’t think it would be a problem throwing him [into the setup role], but in a perfect world, we’d like to ease him into it,” Scioscia said. “I hope he has an any-inning role because he has a power arm that would be a welcome addition to the bullpen.”

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Throw-in

Right fielder Vladimir Guerrero, relegated to designated hitter while he recovers from a torn chest muscle, began a throwing program Monday, playing catch from 75 feet “with some good arm speed,” Scioscia said.

Guerrero will move from catch to long toss over the next few weeks, but Scioscia doubts he’ll be ready to play outfield by the next series in a National League park, at Arizona on June 26-28.

“His legs are OK and he’s been shagging balls, but we have to see how his arm responds,” Scioscia said. “It would be tight. He’s too important to us to try to force something just for that series.”

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mike.digiovanna@latimes.com

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ANGELS TONIGHT

AT SAN FRANCISCO

When: 7:15.

Where: AT&T; Park.

On the air: FS West; Radio: 830, 980, 1330.

Pitchers: Sean O’Sullivan vs. Jonathan Sanchez.

Update: O’Sullivan, a third-round pick from Grossmont College in El Cajon, Calif., in 2005, started the season at double-A Arkansas, where he was 1-2 with a 5.30 earned-run average in three starts before moving to triple-A Salt Lake, where he was 5-2 with a 6.02 ERA in 10 starts. The hard-throwing right-hander was 16-8 with a 4.73 ERA at Class-A Rancho Cucamonga last season. Sanchez gave up only one run and three hits in 5 2/3 innings of his last start, at Arizona on Thursday, but he struggled with his control, walking a career-high seven.

-- Mike DiGiovanna