To make room for Downs, the Royals optioned right-hander Casey Coleman to Triple-A Omaha.
Downs, 38, was released by the Chicago White Sox on Thursday after going 0-2 with a 6.08 ERA in 38 games. Not good numbers.
"They were pretty good against left-handed-hitting," Yost said.
Indeed, Downs this season held left-handed batters to a .224 (11-for-49) average and his career mark against lefties is .219 compared to .273 versus right-handed batters.
Downs said that the 13 unintentional walks he issued in his 23 2/3 innings for the White Sox this year were his main problem.
"Walks -- that does it. Not throwing strikes," Downs said. "When you're facing one guy and you don't get him out and he ends up scoring -- there's your numbers. ... Your ERA is shot for the year."
Downs joins left-handers Francisley Bueno and Bruce Chen in the bullpen, neither considered to be a situational lefty. Nor was Tim Collins, now sharpening his skills with Triple-A Omaha, a lefty specialist.
In fact, Yost couldn't remember having what he considered a situational lefty in his Kansas City tenure.
"I don't know that we've had one," he said.
Downs, in 13 years, has a 38-38 career record and a 3.57 ERA in 602 games (50 starts). Prior to the White Sox, he pitched for the Blue Jays, Angels, Expos, Cubs and Braves. From the University of Kentucky, he was a third-round pick by the Cubs in the 1997 First-Year Player Draft.
"He's a good left-on-left guy," Yost said. "He's a durable guy that can use three or four days in a row to come in and get an out or two outs."
Yost wasted no time getting Downs into action. He pitched the seventh inning of Sunday's 4-1 loss to the Indians, retiring all three batters. That included two lefties, Jason Kipnis (flyout) and Michael Brantley (groundout), and switch-hitter Asdrubal Cabrera (strikeout).
Downs has morphed into the situational lefty role in recent years.
"The good thing about that role is that the game is always on the line at some point when you come in," Downs said. "When you're pitching at this level, that's what you want. You want to come in when the game's on the line. That's what's fun. That's what you play for."
Coleman departed after his only outing since being brought up on Wednesday for the second time this season. He gave up a run, two hits and a walk in a one-inning appearance in Saturday night's 7-3 loss to the Indians.
Bringing on Downs at age 38 after last Monday's signing of 42-year-old outfielder Raul Ibanez is an indication of the Royals' determination to reach the playoffs this year.
"Well, yeah, we're trying to improve our club any way we can," Yost said.