It is feared that the Miami Heat forward could have blood clots on his lungs, which would force him to miss the rest of the season. Bosh reportedly is set to undergo further testing at a hospital on Friday.
James, who joined the Cleveland Cavaliers this summer after spending four seasons with Bosh in Miami, tweeted that his heart is "heavy right now thinking about @chrisbosh health!"
In the first tweet, James references former NBA player Jerome Kersey, who recently passed away at the age of 52.
Bosh re-signed with the Heat on a five-year, $118 million contract in July. Over 44 games this season, he has averaged 21.1 points and seven rebounds while earning his 11th All-Star appearance.