Halfway to STX the flight received an ACARS to return to ATL. Crew asked why, told we will discuss on ground. Crew threatened to return to MIA unless told what was going on. Issue with the engine swap in TYS the night before- parts were found left off after the aircraft had operated TYS-ATL and just before being handed off to Havana center.

I'm sure ASAP will take care of it!