"This was planned all along," Melvin said. "He's played every day."
Donaldson has started 114 of the A's 116 games at third base, where Alberto Callaspo started on Monday, with Brandon Moss -- who made his first two career appearances at the position earlier this year -- serving as his backup.
Donaldson, meanwhile, was seen as the backup to catcher Stephen Vogt, with Melvin willing to risk the possibility of losing his DH rather than use a player with little to no experience behind the plate, while Derek Norris remained unavailable due to back spasms.
"My first option would be to take Josh out of the DH spot and pinch-hit in the catcher spot," Melvin said. "I don't even want to think about that, to tell you the truth, but that would probably be my first course of action."
Outfielders Josh Reddick and Yoenis Cespedes were the only two to play in the field in all four games of the series, and Melvin said he'd likely give Cespedes a start at DH on Tuesday against the Astros.
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.