Right-hander Nick Tepesch was designated for assignment to make room for Healy on the 40-man roster.
Healy started at third base and went 0-for-4 in an 8-7 loss to the Blue Jays. Healy batted ninth, one spot behind Jake Smolinski, who made his second start of the season in center field -- something Melvin said the team will do more regularly moving forward.
"It's going to be tough on some of the guys," said Melvin, whose club entered Friday at 38-51. "But we've put ourselves in this position as a team. With our record, we have to look at some other guys and see if they're going to be pieces for us in the future."
Melvin said Valencia (.304 average and 12 homers in 67 games) will receive more at-bats against left-handed pitching at first base, which could affect Yonder Alonso's playing time. Oakland's already relegated Billy Butler primarily to a bench role this season and Coco Crisp, who has a $13 million vesting option for 2017 if he plays in 130 games or makes 550 plate appearances, will cede center field playing time to Smolinski.
"This will have a little bit of a trickle-down effect for everybody," Melvin said.
Healy arrives after a strong performance in this week's SiriusXM All-Star Futures Game, going 2-for-3, and a powerful season in the Minors, hitting .326/.382/.558 with 14 home runs and 64 RBIs in 85 games between Double-A Midland and Triple-A Nashville.
A natural first baseman, Healy will primarily play third, and worked on individual fielding drills with A's coach Ron Washington prior to Friday's game. Healy said he doesn't have a preference in terms of what position he plays.
"I found out in this game that having a preference doesn't really help your cause," Healy said. "So I'm here to help out any way I can."
The 24-year-old found out Thursday night he'd be making his Major League debut on Friday, but not before the coaching staff at Triple-A Nashville had some fun at his expense.
"They convinced me that I wasn't playing hard enough and that I was going to get benched for a week because I looked like I was tired," Healy said.
"I was sitting there arguing with them saying I didn't want a single day off, let alone a week. Then our hitting coach came in and said if you do that in front of Bob Melvin tomorrow, you're going to have a big issue. And I said 'Excuse me? Who?'"
Burns, 26, enjoyed a breakout rookie season last year, hitting .294 while stealing 26 bases, but he has lacked the same consistency in his sophomore season.
"I think it's more confidence than anything," Melvin said. "They made some adjustments against him. He was trying to do some things a little bit differently. Whether it was looking for more pitches, getting into some better counts, things he wasn't used to doing.
"Any time you have to make some adjustments, you have some struggles with it. Based on the complement of guys we have here, we felt like it was the right thing to get him down there and work on that at Triple-A."
Melvin was asked if the balancing act of involving younger players moving forward while finding playing time for veterans was a difficult task.
"Very much so," he said. "It is difficult. Again, that's kind of where we are right now as a team. If we're 10 games above .500 right now, we wouldn't be looking at this. It does, it presents its challenges. You try to stay as communicative as you possibly can when they're going to play."
Hill to start on Sunday
Melvin said lefty Rich Hill will start on Sunday against the Blue Jays. Hill was scratched from a scheduled start on Friday with a blister, which Melvin said healed fine. Hill is 9-3 with a 2.25 ERA in 13 starts, and missed all of June with a groin strain.