"A little sore right now, so we'll probably let it heal up for a little bit and then strengthen and then get him throwing," Melvin added. "I don't think this will be a long-term deal."
Dull suffered a right knee strain during his one-inning appearance in Friday's 3-2 win over the Red Sox, feeling a pop during his first pitch to Mitch Moreland. He was scheduled to get an MRI on Sunday morning.
"I pushed off and felt something," said Dull, who retired Moreland to end the 10th inning. "Every pitch after that, it was really painful to push off."
Dull said a similar incident occurred during his freshman season at UNC-Asheville eight years ago, "but it didn't feel as severe as it did [Friday] night," he said.
The right-hander was diagnosed with hamstring tendinitis and irritation to the meniscus at the time, saying he was fully recovered six to eight weeks later; it's notable, however, that he had a full summer to recover, since the injury happened at the end of the season.
"We're hoping it's not a big deal, but we felt like it would probably be a week, either way," A's manager Bob Melvin said. "Hopefully this is just a minor thing that resolves itself within a week and we get him back sooner than later."
Dull had an exceptional 2016 rookie season, pitching to a 2.42 ERA with 20 earned runs allowed in 74 1/3 innings. This year, he's already allowed more than half that total, yielding 11 earned runs across 15 2/3 innings; but he pitched a clean inning Friday, compounding his frustration.
"I have a knack for weird injuries," Dull said. "I try to stay as healthy as I can, but weird things just happen. I finally felt like I was starting to get back to normal and then something happens."
Axford, meanwhile, is readying for his first appearance of the season, having missed 42 games with a shoulder strain. He made three rehab appearances for Triple-A Nashville, striking out four in a combined 3 1/3 scoreless innings.
"To get another fresh arm today is good," Melvin said. "Obviously having to put Ryan on the DL hurts us. He's been a big factor for us.
"[Axford] might have to be thrown right back into the fire. Usually you want to give him one so-called soft landing, but I don't know if that will be the case with him."
Mengden activated, optioned
Right-hander Daniel Mengden, who was expected to compete for a rotation job in Spring Training before suffering a right foot injury that required surgery, was reinstated from the disabled list Saturday and optioned to Nashville.
Mengden was 2-9 with a 6.50 ERA in 14 starts for Oakland last year.
"You can't have enough depth," Melvin said. "We've seen with the injuries this year, our top four starters have gone on the DL at some point in time or another, so it's nice to have that kind of depth. He is definitely part of our equation when you're looking at starting pitchers. I'd actually be surprised if we didn't see him at some point in time this year."
• First baseman Yonder Alonso (left knee) will likely be rested through Monday's off-day.