Bobby Nightengale | The Minnesota Star Tribune

KANSAS CITY, Mo. – The Twins are already without their All-Star center fielder, Byron Buxton, and now they will be without All-Star starting pitcher Joe Ryan for the next two weeks.

Ryan is headed to the 15-day injured list after a medical resonance imaging exam Thursday revealed a mild left glute strain. The Twins haven’t formally announced a starter for Friday in Milwaukee, but Zebby Matthews was pulled from his scheduled start Thursday at Class AAA St. Paul.

There was no immediate timetable for how long Ryan will remain out, but manager Derek Shelton was encouraged that it was a mild strain.

“When I hear the word mild, I take a deep breath in and feel good about it,” Shelton said. “It wasn’t something significant. Any time I hear mild, I feel like we’re in a pretty good spot.”

Ryan returned to the Twin Cities on Wednesday to be evaluated by Twins medical director Chris Camp after he hasn’t felt like himself for almost a month. He posted a 7.59 ERA in his last four starts, giving up 31 hits and 18 earned runs in 21⅓ innings while striking out only 16.

There was no immediate timetable for how long Ryan will remain out, but manager Derek Shelton was encouraged that it was a mild strain.

“You don’t want something to be wrong, but you also want to make sure that when your player is feeling something, that we’re able to attribute it to something specific,” Shelton said. “When you know specifically it’s isolated to an area, yeah, that makes you feel better.”

Matthews was optioned to Class AAA after the Twins acquired Dean Kremer ahead of the trade deadline. Matthews has a 4-8 record and a 5.22 ERA in 14 starts this season, striking out a career-high 10 batters in five innings against the Seattle Mariners during his last outing.

Byron Buxton progressing slower than expected

Buxton is eligible to return from the 10-day injured list this weekend in Milwaukee, but he has yet to resume baseball activities after he aggravated his right hip impingement.

He received an unspecified injection last week.

“It’s a little slower than what I expected, but I’m starting to pick it up a little more, do a little more things as far as loading, isometrics and weight training stuff, lifting,” Buxton said. “All that’s going pretty well. Hopefully, relay that over to the field over a couple of days.”

This is Buxton’s second stint on the IL because of his right hip, a nagging injury that bothered him in mid-May, then flared up again at the end of June.