Head athletic trainer Lonnie Soloff said the strain, which was revealed by an MRI after Miller was unable to throw Monday, is "mild to moderate" and will require three to four weeks of rehab.
The forearm strain, Soloff said, is unrelated to the elbow strains that kept Miller out of the second half of the '05 and the better part of the '06 seasons. But, like those injuries, it was encouraged by Miller's sidearm delivery.
"It's certainly related to how Matt throws the baseball," Soloff said. "His arm slot exposes his elbow and forearm to more stress."
Losing Miller puts a little bit more stress on the Tribe's bullpen, because the club was counting on him to be a situational right-hander in the late innings.
At the least, though, Lara and Mastny have some Major League experience under their belts.
"They both have a lot to offer," manager Eric Wedge said. "The experience they gained is a definite plus."
Wedge said the fact that Mastny is right-handed and Lara is left-handed will play into the decision, but it won't be the overriding factor.
"We're going to look at how they complement the other guys in the bullpen," Wedge said.
Beyond their opposite handedness, the 26-year-old Lara and the 25-year-old Mastny bring different styles and strengths to the Indians.
Both experienced their first exposure to the big leagues last season, though Mastny's time was a little more prolonged and a little more meaningful. After getting called up from Triple-A Buffalo in late July, he compiled a 5.51 ERA over 15 appearances, and, for a stretch, was dominant in the closer's role, saving his first five opportunities before Major League hitters caught up to him in September.
"If you look at last year and the way he did late in games, that's rare," Wedge said. "He's a strike-thrower, and he's got some spin. You can count on him."
Lara was a September callup who appeared in nine games, giving up just one earned run over five innings of work.
"Lara is deceptive," Wedge said. "He pitches with no fear, he's aggressive, and he has a pretty good fastball. You put that together, and it's a pretty interesting skill set for a left-hander."
Mastny and Lara have both had impressive camps. They came into Tuesday's game against the Astros, during which they were both scheduled to pitch, with the two lowest ERAs on the staff.
Lara had compiled a 0.79 mark over 11 1/3 innings in eight appearances, striking out 10 batters. Mastny had put up a 1.50 ERA in 12 innings over nine games, also striking out 10.
Wedge said he expects a decision to be made between Mastny and Lara in "the next day or two." The same goes for the Indians' other roster decisions -- whether they'll keep first baseman Ryan Garko or an extra outfielder, and whether they'll carry Mike Rouse or Luis Rivas as the utility infielder.
The rest of the bullpen is set with closer Joe Borowski, right-handers Rafael Betancourt, Roberto Hernandez, Fernando Cabrera and Jason Davis and left-hander Aaron Fultz. But only Borowski will have a clearly defined role by Opening Day.
"We won't necessarily establish roles at the outset of the season," Wedge said. "We'll base it on who will give us the most versatility."