Simon, who had been sidelined since June 13 with a strained right hamstring, is 1-1 with a 3.14 ERA in five games this season.
Rapada appeared in 22 games with Baltimore, going 0-0 with a 7.30 ERA. He held left-handed batters to a .114 batting average, but right-handers were hitting .692 (9-for-13) against him.
Most of all, however, the Orioles had no room for a situation pitcher in an overworked bullpen that has logged plenty of innings this month. In 23 games in June through Tuesday, only once did a starter go at least seven innings -- Jake Arrieta on June 10 against the Tampa Bay Rays.
"Obviously, I think a lot of the move was dictated by we need some length in the bullpen," manager Buck Showalter said. "And Clay, he did what he was designed to do statistically with left-handers, but we just weren't able to carry (a specialist), weren't in position with our starters not going as deep as we hoped."
Rapada signed a minor league contract with the Orioles over the winter. He previously pitched for the Chicago Cubs, Detroit Tigers and Texas Rangers. He made Baltimore's 25-man roster out of spring training because the team was short on left-handed relievers, but he became a luxury the Orioles couldn't afford to keep.
"If you are in the right situation, your starters are going deep enough, you are able to carry someone like him," Showalter said. "We are just not in that position right now."
If the 30-year-old Rapada ends up becoming a free agent, there's a chance he could latch onto a team looking for someone to get left-handed batters out in the late innings.
"You can look at the ERA overall, but you look at the numbers in the situation he is designed to do and you can see why he has been a chip for people that are playing late into October," Showalter said. "So it will be interesting to see how it develops."
Simon has the ability to start but will be used initially in long relief.
"If the starter doesn't go a lot of innings, I can go four or five innings to help the team win the game," he said.
Showalter said Simon, who had 17 saves last year, might also be used in the late innings.
"Like all this," the manager said, "it's how deep we can go with the starting pitching."