"He's pitched the last two days," Showalter said. "He'll be in uniform and available tomorrow."
The Orioles acquired Miller from the Red Sox today for minor league pitcher Eduardo Rodriguez, strengthening a bullpen that's already been solid.
"He's just another good piece," Showalter said. "He goes with some other good pieces down there. He gives us more depth down there, keep passing the load around.
"I think he's a little more than just a left-handed arm. You look at left-handed relievers, the ideal ones are the guys that you've got left-right-left and you can leave them in there for the right."
That's true with Miller. As I've pointed out a few times already, left-handers are hitting .150 against him and right-handers are batting .180.
Showalter wasn't pressing for a trade.
"I've been fine where we are," he said. "What's tough is now someone's got to come out of there."
T.J. McFarland, Brad Brach and Ryan Webb have options. Keeping McFarland would give the Orioles four left-handers in the bullpen.
"If we do have to make a move, they're not going away," Showalter said. "They're going to (Triple-A) Norfolk. And keep in mind that the rosters expand in four weeks, roughly.
"(Miller) has done it at a high level, in pressure situations and done it in the American League and the American League East. There's a lot of factors that lean toward him being good. There's always a period where guys make a transition to a new club, but I feel confident bringing guys into our locker room.
"Not excited about losing Eduardo, but you've got to give up quality to get quality."
Showalter expects to stay with 12 pitchers, "but it could change," he said.
"A lot's got to do with Ubaldo (Jimenez). He's pitching in Norfolk Sunday or Monday. Trying to get him closer to 100 pitches. Hopefully, by the end of the day we'll know exactly what's coming or going."
Miller arrives tonight. How long he stays, as a pending free agent, is another matter.
"We know that Miller is going to be a free agent," Showalter said. "We know that the Red Sox like him and other teams will like him, but we think he can impact our ability to make the playoffs and we're willing to take that risk."