Pennington, a versatile veteran, figures to make the team in a reserve role while youngsters Gregorius and Owings battle for the starting job.
While neither Gregorius or Owings has much, if anything, left to prove in the Minors, that's the likely destination for whichever one loses out in the battle for the starting job.
"I think it would be hard," D-backs general manager Kevin Towers said of keeping both young players at the big league level. "I don't think it's good for their development. They're both everyday type players and they're ready to be everyday type players. I would just hate for one of those kids to be sitting on the bench and playing once or twice a week. They'd be better off being in Triple-A, getting at-bats and getting ready in case there's an injury."
Gregorius saw more big league time than Owings, joining the D-backs in April last season, while Owings was a September callup.
Gregorius started off hot at the plate, but he slumped as the year went on, with his monthly batting average from April to September going .407, .293, .241, .234, .197 and .182.
Owings, meanwhile hit .330 for Triple-A Reno of the Pacific Coast League and .291 for the D-backs in September.
"To me, the favorite has to be Didi," Towers said. "He was our shortstop last year. For a rookie, I thought he played very, very well. Great defender. He got off to a real hot start. The league probably caught up with him a little bit at the end. He probably got a little bit tired.
"Chris Owings was the MVP of the PCL and performed very well in September. I think 'Penny' knows he's in the competition, but these are two guys that are probably more of the future, and he's going to be supporting one or the other or maybe both. But it should be very competitive, and I look forward to watching them go out and play and compete."
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.