Marlon Starling
vs.
Mark Breland
CANCELED

The fight between Marlon Starling and Mark Breland, which was to take place on April 15, 1989, was canceled due to a shoulder injury from Starling. Breland instead fought Rafael Pineda on April 22, but the fight was not shown on HBO, airing instead on ABC's Wild World of Sports.

I could find no evidence that the other scheduled fight between Julio César Chávez and Rodolfo Aguilar took place either.

According to that day's schedule in The New York Times, the fight was replaced by an airing of The Quick and the Dead, starring Sam Elliot.