"I've seen Dontrelle pitch since he was a rookie like a lot of us have, and he's always had a very unique delivery, so it's almost impossible to say what he's doing in his delivery that would cause those control issues," said Dipoto, who added that Willis' fragile fingernail on his left middle finger didn't warrant a trip to the 15-day disabled list. "Whether it's mechanical, I can't tell you, but only he can sort that out."
After five uneven starts, he was relegated to the bullpen by recently dismissed manager A.J. Hinch. Willis made his first appearance as a reliever in Saturday's 14-1 loss to Los Angeles, allowing three runs and plunking Casey Blake in the sixth inning.
"The guy has had a wonderful career," Dipoto said of Willis' impressive Major League beginnings with the Florida Marlins. "We gave him this opportunity in the effort to try to resurrect him ... and just catch lightning in a bottle. After talking to him [Sunday morning], it didn't particularly work this time around."
"We all respect Dontrelle," interim manager Kirk Gibson added. "He's a max-effort guy. He's been really good [in the clubhouse]."
Jordan Norberto was recalled to assume Willis' 25-man roster spot. Norberto, the D-backs' first true lefty since he was optioned to Triple-A Reno on April 21, was available to pitch Sunday, one day after Gibson called on five of his seven relievers to pitch.
Norberto, 23, made the big league club for the first time out of Spring Training but allowed three runs in three innings before being sent down. The lefty, who totes a 90-plus mph fastball, curveball and changeup, was told to throw more strikes.
"I've been working on my control," said Norberto, who went 3-0 with a 3.07 ERA for the Aces but did issue 19 free passes in 21 appearances. "The first time I got here I wasn't ready. But I'm ready to go [now], and I'm going to keep working hard."
Norberto wasn't given any guarantees as to how he would he utilized in his second big league go-around.
"We'll have to try to put him in good situations, yet he may find himself in one that's not so favorable," Gibson said. "We'll check him out today. He's confident. I know he's worked hard."