Slaten, 28, was 0-3 with a 4.55 ERA in 27 2/3 innings this season. He received an MRI on the knee but will not know the results for at least three days.
Right-hander Chad Qualls was also unavailable Friday night after he strained a muscle in his lower back. D-backs manager Bob Melvin said that with a banged-up bullpen, which currently has three relievers already on the disabled list, there was an imminent need for another reliever.
The D-backs now have no healthy left-handed relief pitchers on the 40-man roster and have six players on the DL.
"The bullpen needs a bit of a rest," Melvin said. "It's a little against the grain being that we're one day after the break. We're a little taxed, but that's the way it goes."
Saturday would have been Buckner's day to start for the Sidewinders, so Melvin said he would be available for long relief.
The D-backs likely will make another roster move on Sunday. Catcher Chris Snyder said he is ready to come off the disabled list after going 2-for-5 with a home run in a rehab stint for Class A Visalia Friday night.
Snyder, who severely bruised a testicle when a foul ball bounced up and hit him June 30, caught all nine innings and felt no discomfort.
"It went good," Snyder said. "I caught the full nine, got five at-bats -- everything held up good. It was a good test last night. To put it this way, in the second or third inning, there was a foul tip that hit the ground and came up and got me again. Everything held up good. If that's fine then we're good to go."
Snyder joked that the big test for him was just to be able to catch a full game and then be able to walk afterward. He said he was able to go through all his stances and block balls in the dirt.
"Something like that is crazy, you never expect it to happen, just deal with it," Snyder said. "I don't want anyone to ever experience it. It was the worst pain I've ever felt in my life, hands down."
Melvin said the club has not decided whether to keep three catchers or to send down a reliever to make room for Snyder, but with such a strapped bullpen, the club is likely to send down Robby Hammock, who owns a .171 average through 16 games.