Kenley Jansen placed on disabled list with irregular heartbeat

Kenley Jansen was healthy enough on Friday to throw a bullpen session but still landed on the disabled list.

Jansen, who was hospitalized earlier in the week because of an irregular heartbeat, is on blood-thinning medication to prevent a clot or stroke. If struck by a baseball, Jansen could bleed excessively.

The hard-throwing reliever will be on the medication for three weeks, after which he should be able to return to competition.

Trainer Stan Conte called the measure precautionary, saying it was very unlikely Jansen would have suffered a clot. The decision to take the blood-thinning medication and land on the disabled list was Jansen’s.

“Health first, baseball second,” said Jansen, who has a 3.65 earned-run average in 34 games.

Jansen was replaced on the active roster by Josh Lindblom, who was recalled from double-A Chattanooga.

Jansen underwent a series of tests over the last couple of days, but Conte said the cause of his irregular heartbeat was never discovered.

“Kenley is no longer having a problem and may not have one again the rest of his life,” Conte said.

Jansen said he noticed something was wrong at around 3 p.m. on Tuesday.

“It made me feel tired,” he said.

He kept quiet and closed out the Dodgers’ 3-2 victory over the Colorado Rockies that night. When the condition didn’t subside after the game, Jansen alerted the medical staff.

Jansen will be able to work out while on the disabled list, but he will have to make some adjustments to ensure his safety. He won’t be allowed on the field during batting practice and can’t sit in the dugout during games.

He will be able to sit in the bullpen during games, provided he wears a helmet.

Blake nearing return

Casey Blake struck out twice in four at-bats in his first game with Class-A Rancho Cucamonga on Thursday, but Manager Don Mattingly said the veteran third baseman could be activated after only one more minor league game.

If Blake is activated Saturday, he could take the roster spot of Juan Uribe, who remains sidelined with a tight groin muscle. Uribe has missed the last five games.

The Dodgers have been mindful of giving their rehabilitating players plenty of at-bats in the minor leagues before activating them, but Mattingly said Blake could be an exception because he isn’t expected to return as an everyday player.

Blake, who turns 38 next month, has been on the disabled list three times this season.

Short hops

Carlos Monasterios, who pitched in 32 games with the Dodgers last season, underwent Tommy John surgery earlier this month. Monasterios was limited to only one game this season, which was for triple-A Albuquerque. He is expected to be out for 12 to 16 months.... Rubby De La Rosa will switch uniform numbers from 50 to 41 when he makes his next start on Sunday.

dylan.hernandez@latimes.com