A health concern of a top CBS exec is creating an unexpected dramatic twist ahead of the all-important upfront season.

Entertainment President Glenn Geller told staff in a memo he’d suffered a “mild heart attack” and would be taking time off until the end of May.

Geller is responsible for not just the primetime schedule, but also daytime and late night, including the surprisingly resurgent “The Late Show With Stephen Colbert.”

Networks currently are picking pilots to show at the May upfront, when advertisers see what’s coming down the pike before they commit to spending more than $70 billion.

CBS Chief Executive Leslie Moonves, who’s typically very involved in choosing shows, sent a memo telling the rest of Geller’s staff that they’d have to up their game and fill in for the boss until he returns.