The 51-year-old father of Lindsay Lohan was rushed to hospital on Friday morning after complaining of chest pains at his rehabilitation facility in Florida. He was treated for the same issue in November last year.
Doctors are believed to have found blood clots and have started him on a course of medication to break them up, meaning that he is unlikely to need surgery.
However, TMZ claims that Michael will be kept in hospital for several more days as his condition is monitored.
Michael previously underwent heart surgery early in December. The operation - which involved fitting a stent - was postponed after blood clots were discovered in his lungs.
More clots were found when he collapsed at an Alcoholics Anonymous meeting later that month.
Michael first had emergency surgery on his heart in February 2011.