Rosin, a 24-year-old Minnesota native, has since adjusted to the move, just as he adjusted to an ominous start to his 2013 season.
"He loaded the bases with nobody out," Reading manager Dusty Wathan said. "And then he struck out the side."
He allowed two runs on five hits and two walks in his four-inning Reading debut at New Hampshire. He also struck out six batters. The next time out he pitched six shutout innings and allowed just three hits and a walk against Erie. He produced another quality start Thursday against Richmond, allowing three runs on five hits without walking a batter in six innings.
Schierholtz was released and signed with the Chicago Cubs after last season, so the success or failure of the Pence trade will hinge on the development of Joseph, who is at triple-A Lehigh Valley, and Rosin, the lesser of the two prospects in the deal.
It's too soon to know exactly what the Phillies have in Rosin, but his first six starts - three this season at Reading and three last season at Clearwater - have been positive.
Throwing strikes has been his greatest weapon. His four-year professional WHIP (walks and hits per innings pitched) is 1.166, including 1.000 this season. He has struck out 192 batters, walked 56, and allowed 159 hits in 1841/3 career innings.
"He's got a good heavy fastball with sink, a good slider and he's got a good change-up too," said Gorman Heimueller, the Phillies' minor-league pitching coordinator. "He's got three good pitches and throws strikes."
The Giants selected Rosin from the University of Minnesota in the fourth round of the 2010 draft and used him more as a reliever than a starter.
Heimueller said it's possible Rosin ends up being a reliever, but the Phillies want him to get more consistent work as a starter for the time being.
"I was a late-inning reliever for most of last year," Rosin said. "I think we had a starter go down and they tried me out there. I was drafted as a starter and every year since being drafted I've done both. It's kind of nice going into this year knowing I'm going to be a starter and keeping focus on that."
Rosin grew up in Minnesota and was drafted by the Twins out of high school, but he opted for college instead.
"It was tempting," he said. "But I don't think I would have been anywhere near ready emotionally if I had just gone straight into pro ball. I'm really thankful I was steered into college, where I could find myself as a person before leaving home and being totally on my own."
Rosin said there is a bias against baseball players from the north that he is trying to dispel.
"I've always had a chip on my shoulder," he said. "I've had teammates tell me that no good players come out of the north and stuff like that. I've always wanted to prove those people wrong, especially since I'm from a northern college. There is a lot of bias against that."
Contact Bob Brookover at email@example.com. Follow on Twitter @brookob.