Anderson, who has one hit in his past 19 at-bats and is in a 3-for-36 slump, has a pulled left hamstring, which has curbed his speed.
Right-hander Frohwirth, the International League leader with eight saves, will "complement Kevin Hickey on the other side," said manager John Oates.
His role will be to face right-handed hitters in tough middle-inning situations, although Oates said that "there are times when he'll have to go a couple of innings."
Frohwirth, 28, made his Orioles debut last night, retiring the Cleveland Indians in order in the eighth inning.
With the Red Wings, Frohwirth was 1-3 with a 3.65 ERA, allowing 17 hits and five walks in 24 2/3 innings. The Orioles signed him last winter as a minor-league free agent out of the Philadelphia Phillies system.
He filled the bill as someone with previous major-league experience. In four stints with the Phillies spanning a little more than a season, he was 3-3 with a 3.95 ERA.
"I've been throwing the best I ever have in my life," said Frohwirth, a submarine pitcher who is most effective against right-handed hitters. "I'm not really surprised. I thought this would happen soon."
Frohwirth said he signed with the Orioles over several other offers because "no one else understood as well as the Orioles that I could play in the big leagues."
In his most recent appearance, three nights before he was called up, he went 3 1/3 innings. Since the Orioles' current opponent, the Cleveland Indians, and their next one, the Boston Red Sox, have predominantly right-handed lineups, Frohwirth may see plenty of action in the next few games.
"He fits the situation we have right now," said Oates, who wanted 11 active pitchers. "And he was doing better than anybody down there."
Frohwirth pitched only 2 2/3 innings during spring training, lost a game and had a 10.13 ERA.
Third baseman Craig Worthington is recovering well from his pulled hamstring and may be ready to rejoin the team when his 15-day term on the disabled list ends.
"I've been taking ground balls and hitting for a couple of days," said Worthington, who was hurt in Detroit last week and placed on the DL May 22. "No running yet. That's the real test."
He said he is pleasantly surprised by the recovery since, when it happened, he "thought it was a lot more serious than it has been. There was a little pop. When I had that in the other [left] leg, I was out quite a while.
"I hope I'll be ready in another week."
Encouragement for Milacki
Bob Milacki absorbed his second defeat Monday night but felt that he "pitched better than in the past and my arm felt strong even at the end."
Oates plans to keep Milacki in the starting rotation with Ben McDonald disabled.
"I wasn't gassed in the heat," said Milacki. "But maybe the rest of my body was getting winded, not my arm. I haven't pitched in that kind of weather in a while. It takes something out of you."
Milacki walked only one in five innings, an indication that he was on his game. He pointed out that of his 14 walks this season, eight have come in two poor outings.
"Sometimes I start to jump out at the plate and get a little wild," he said. "I just miss the strike zone."
The Oriole Advocates will hold their eighth annual Oriole Hall of Fame luncheon June 28 at the Hyatt Regency to honor this year's inductee, Hal "Skinny" Brown. Tickets are $25 a person or $250 a table of 10 and are limited to 400 people. For more information, call 252-0268, 252-6769 or 655-4204. . . . Openings remain for the June 30-July 5 week for special prospects at the Cal Ripken Baseball School. Enrollment is limited to players 15 to 18 years old. For more information, call (301) 447-5296 (day) or (301) 791-3512 (any time). . . . Oates lost 9 pounds in his first four days as manager. He weighed 174 yesterday, the least he has been since 1977, when he was catching for the Los Angeles Dodgers.