BOSTON -- When the Royals arrived in Boston, their bullpen was well rested. After three games at Fenway Park, the relief corps needed a fresh arm.

So before Monday's game against the Red Sox, the Royals called up right-hander Vin Mazzaro from Triple-A Omaha. He'll take over the spot vacated by right-hander Jeremy Jeffress, who was optioned to Double-A Northwest Arkansas.

"I need an arm," manager Ned Yost said.

Jeffress pitched two-thirds of an inning in Sunday's 8-6 loss and gave up three hits, a walk and two unearned runs while throwing 28 pitches. In eight games in the Majors this season, Jeffress has yet to be charged with an earned run while pitching 6 2/3 innings.

Jeffress was sent to Double-A because Northwest Arkansas' schedule ends on Sept. 3 and he could return to the Royals then instead of waiting the usual 10 days before being recalled.

Yost ruled out using relievers Aaron Crow and Tim Collins on Monday because they'd both pitched in all three previous games vs. the Red Sox.

Mazzaro was rested, having not pitched for Omaha since last Tuesday. For the Storm Chasers, he had a 2-2 record, five saves and a 3.63 ERA in 22 games, including eight starts.

"He's been throwing fine in relief, throwing strikes. He's been pitching the back end," Yost said.

This is the fourth time that Mazzaro has been recalled from Omaha this season. In his previous three stays with the Royals, he was 3-3 with a 6.12 ERA in nine games, six of them starts.

Before facing the Red Sox, Royals starters had pitched six or more innings in 10 of 14 games. Two of the four short starts came against the Rays in two of the first three games of the club's road trip, but there was a day off before the Boston series.

"[Bruce] Chen went six, which was good, but then [Jeremy] Guthrie 2 2/3 and then five innings [by Will Smith] yesterday," Yost said, noting the Guthrie game went 12 innings. "That's why it was important for Will to get us through six yesterday; that would've made a big difference, but he couldn't do it."

So the call went out for Mazzaro.

Bullpen hitting its stride during August

BOSTON -- The Royals are bullish on their bullpen.

In the 25 games prior to Monday's series finale against the Red Sox, the Royals' relief corps had a 1.94 ERA, charged with just 16 earned runs in 74 2/3 innings. In that span, only two other Major League bullpens had better ERAs -- Tampa Bay (1.08 ERA) and Atlanta (1.43 ERA).

"We've been throwing well for the last month or so. Hopefully we can keep it up in September and finish strong," right-hander Aaron Crow said.

The upturn has followed better and longer performances by the Royals' starters.

"They've been throwing the ball a lot better, so it's nice," said Crow. "It's given us a break, so when we come in there, everybody's fresh and we're winning a lot more games, too."

There had been concern earlier in the season that the bullpen was racking up so many innings that the young pitchers might be worn down late in the season. Crow, along with Greg Holland, Tim Collins and Kelvin Herrera, is over the 50-inning mark, but he doesn't feel fatigued.

"No, not at all," Crow said. "I think everyone's done a good job of keeping their bodies in shape and ready to go. I feel strong, and I think it's safe to say pretty much everybody down there does."

Crow was an All-Star as a rookie in 2011 and this is his second full year of relief work. During Spring Training, the Royals tried converting him into a starter -- his role at the University of Missouri and in the Minor Leagues -- but that idea was abandoned.

He enjoys the bullpen, but what if the Royals again broached the starting possibility?

"I'd be open for it. I'd try anything," Crow said, "but right now I'll stick to the bullpen until I get told otherwise. I feel comfortable doing either one, so we'll just see what happens."

Crown points

• Starter Bruce Chen stepped on a ball during Monday's batting practice and went down, bringing out assistant head trainer Kyle Turner to inspect his right ankle. But Chen got up, smiled, took some razzing and was reported to be fine.

• The four-strikeout game endured by Billy Butler on Sunday was the first in the 817 games of his career.

• The two-error game by shortstop Alcides Escobar on Sunday was just the third of his career; the first two occasions came when he was with the Brewers.

• Of the Royals' last 35 games, 23 will be at Kauffman Stadium starting Tuesday night against the Tigers. Also coming in during the 10-game homestand are the Twins and the Rangers.