The condition is considered "very manageable," sources told The Athletic's Ken Rosenthal.
"Any time you're talking about the heart, we obviously have to be very careful with it," Roberts told reporters, according to MLB.com's Anne Rogers. "And then we're talking about altitude, so that's why we wanted to be proactive and get him back (to Los Angeles). We're holding out hope that it's not too serious, but talking to the trainer right now, his mood is fine, you wouldn't know the difference, and once we get more testing, we'll know more."
Jansen suffered a similar ailment six years ago in Colorado and underwent heart surgery in October 2012. He hasn't had an issue since, according to general manager Farhan Zaidi.
"I think he's had some discomfort with the altitude in the past, but beyond that we don't really know," Zaidi explained. "We're just glad he felt something, got help right away, and he's feeling pretty normal right now from what we understand."
Jansen is one of baseball's best finishers. He's tied for the National League lead in saves with 32 and has amassed 262 during his nine-year career.