Diamond pitched well with Rochester after being optioned by the Twins in early August, as he posted a 2.02 ERA in six starts, including a strong outing in the International League playoffs.
"He did fine," Twins manager Ron Gardenhire said. "I watched some games. His location was better, and his breaking ball was a little sharper. But what he did down there, it has to translate up here, or else it doesn't mean anything. He can't get behind the count and not finish his pitches off."
Right-hander Cole De Vries, who was also officially recalled from Rochester on Monday, will pitch out of the bullpen for now, but Gardenhire did not rule out De Vries joining the rotation this year. Pelfrey is coming off Tommy John surgery and will be monitored the rest of the season, while left-hander Andrew Albers has already thrown a career-high 178 innings after throwing 103 innings last year.
"De Vries is starting in the bullpen, but if we want to slot him in there, we can take someone out," Gardenhire said. "We do have guys who are running up on innings that we're nervous about."