Blackburn lasted just four innings against the Tigers on Tuesday, allowing six runs on eight hits, including two homers. The right-hander has a 4-5 record and an 8.10 ERA in 13 starts this season.
"There was no final straw," Twins assistant general manager Rob Antony said. "It's just a matter of he was struggling. He wasn't getting it done and he doesn't have confidence -- it doesn't look like he was pitching with confidence -- and we just want him to go down and get himself right."
It's been a tough year for Blackburn, who has thrown just one quality start and hasn't gone longer than six innings in any of his outings. He's also battled injuries, as he missed time with tightness in his shoulder in April and was placed on the disabled list with a strained left quad in mid-May.
"It's been tough the whole time," Blackburn said. "It's very frustrating to go out there and not feel effective. It's been a tough couple weeks. Actually, a tough season. Early on, I felt like I was getting tough breaks and that my pitches were good, but after a while, you start to realize that's not really the case. So to go get some confidence back will be key for me."
It's been a continuation of last year, as the sinkerballer has a 7.61 ERA over his last 24 starts dating back to June 27, 2011. Blackburn pitched well early last year -- he had a 3.15 ERA through his first 15 starts -- but his 7.61 ERA since then is the highest among Major League starters.
Blackburn, who signed a four-year, $14-million extension before the 2010 season, has a career 4.79 ERA in 139 games, 131 starts, over six seasons.
Fien, meanwhile, had a 4.30 ERA with 42 strikeouts and 14 walks in 46 innings with Rochester. He has some big league experience, posting an 8.36 ERA in 14 innings over parts of two seasons with Detroit from 2009-10.
"Casey's been throwing very well," Twins manager Ron Gardenhire said. "Today's report said he's pitching as good as they've seen him throw the baseball. His fastball's 91, 92, 93 mph, and he's a competitor. He can field his position; he's very active, very athletic. So hopefully he'll be able to give us some innings."