The Heat was very concerned on Monday about another serious health situation involving Chris Bosh, multiple sources said. The initial belief is that Bosh has blood clotting in his left calf, according to a league source in touch with the Heat's front office, and a Yahoo! report.

Bosh missed Sunday’s All-Star game with a strained calf and returned to Miami for additional tests. A person in touch with the Heat’s front office said the blood clot issue surfaced Monday.

Asked directly if Bosh had been diagnosed with a blood clot in his calf, agent Henry Thomas responded by text: “Too soon for all of that…. Too soon to report that.”

The Heat did not comment.

Yahoo's new NBA site, The Vertical, reported early this morning that "Bosh is expected to meet with doctors on Thursday to determine the seriousness of blood clotting in his left calf and whether the use of blood thinners could allow him to return this season."

Professional athletes on blood thinners typically are discouraged from physical contact. That was the case last year, when Bosh took blood thinners for seven months after being diagnosed with blood clots in his lungs.

All blood clots are serious, but there was no indication that the blood clots had traveled to his lungs, as was the case last year. This clotting was believed to be only in his calf.

Last season, Bosh missed eight games with a calf injury, then returned to play 21 games but was having difficulty breathing to full capacity.

Then, during the All-Star break, he was diagnosed with blood clots in his lungs, causing him to miss the final 30 games of the season. Bosh said doctors suspected that the blood clots began with the calf injury.

Bosh said last summer that he was told there was “a fairly low risk” of another blood clot.

“The recurring risk comes from a hereditary gene," he said last September. "Fortunately, I don't have that. That was one of the dangers I faced earlier, was, 'Am I going to be able to play the game?’ Because if this test comes back that it's a recurring thing, it's going to be a problem.

“Anyone, if they don't take the precautions, it can be a recurring thing, yes. But I'm a lot smarter now. I know about the precautions."

Bosh said last September that he would take preventative measures, such as getting up to walk during flights, stretching his legs, wearing compression socks and taking Aspirin.

"Just the small things," he said, "Making sure after a game that you continue to move, that's the most important thing, keep that blood pumping in the legs. And, as a taller guy, it's a little bit tougher for circulation.

“That's one of the main primary things I think of, is, 'OK, let me make sure that I keep my blood pumping. Let me get up. Let me wear my compression socks.' Nobody likes to wear those, but they can do quite a bit for you if you take the time to wear them."

Bosh said being away from basketball increased his appreciation for the game.

"I know things can be difficult, but that's what drives me now,” Bosh said last fall. “I would rather be doing that than have tubes in my chest."

Bosh did not miss any Heat games with the calf injury but decided on Friday to skip the weekend’s All-Star festivities.

He said Saturday that he was “pretty optimistic” that his calf strain wasn’t related to blood clots, but that he intended to see a physician on Monday.

“As an athlete, we try to keep going with some things, treating it,” Bosh said Saturday. “But it just lingered. And I didn’t feel it was wise to continue to push it, especially with this elongated week that we have off. I just wanted to make sure I was taking the necessary precautions and being a good professional.”

Bosh is tied for 26th in the league in scoring at 19.1 points per game and tied for 35th in rebounding at 7.4.

Bosh, an 11-time All-Star, is in his sixth season with the Heat. He signed a five-year, $118 million deal with Miami in July 2014.

My colleague Ethan J. Skolnick contributed to this report.