MILAN (AP) — The chairman of Fiat Chrysler Automobiles has written a letter to the carmaker’s employees to tell them that long-time CEO Sergio Marchionne’s condition has worsened, preventing his return to the Italian-American company he created through a bold merger.

Chairman John Elkann, heir to the Agnelli family that founded Italian carmaker Fiat in 1899, called Marchionne a “true, rare kind of leader … (whose) impulse never to accept the status quo, never to be satisfied with good enough, has become a core part of our culture within FCA.”

Marchionne is hospitalized in Zurich, where he underwent surgery on his right shoulder three weeks ago. The company has not elaborated on his complications but said he’s not returning to work.

FCA’s board of directors on Saturday named former Jeep brand chief Mike Manley as the new CEO.