Ramirez, who fouled a ball of his shin on Thursday, won't need surgery but will miss the rest of the season with his injury. Ramirez finishes the year hitting .272/.302/.370 with six doubles, a triple and six RBIs in 35 games. He also missed 69 games after suffering a concussion in late May.
"It's sad," Twins manager Ron Gardenhire said. "He was just coming off that concussion thing. It's been a tough year for him with all the injuries he's had."
Deduno had an MRI exam on Friday that showed issues with his labrum and rotator cuff. The Twins remain hopeful Deduno can avoid surgery, and he's scheduled to meet with Dr. Pearce McCarty in Minnesota on Saturday. Deduno, who has been battling biceps tendonitis over his last five starts, has a 3.83 ERA in 18 outings this season.
"Whether or not he's going to take the mound again this year we're going to have to get him looked at first," Twins general manager Terry Ryan said. "At this point in the year we're not going to rush him back by any means."
Hernandez could take Deduno's place in the rotation on Tuesday but will be available out of the bullpen in long relief until then in case he's needed. He has a 5.54 ERA in nine appearances with the Twins this year, and is coming off a forearm strain suffered in July.
"Pedro is in the bullpen right now but we'll work on Tuesday over the next couple days," Gardenhire said.
Mastroianni, who began the year with Minnesota, has missed most of the season with a stress reaction in his left ankle stemming from a foul tip off his ankle late in Spring Training. But he's back up with the Twins after hitting .240/.397/.260 in 15 games at Triple-A Rochester, and started in right field against the Rangers on Friday.
"It's been a long season," Mastroianni said. "It's just nice to be back up here with these guys. It was excited to be told last night I'd be back here. I'm just looking forward to finally being healthy and being able to play again this year."
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.