CHICAGO -- Jake Arrieta, acquired from the Orioles in the Scott Feldman deal, will join the Cubs' rotation and start Friday against the Cardinals, while Carlos Villanueva is headed back to the bullpen.
Arrieta, 27, was sent to Triple-A Iowa after he was acquired, but he did make one start for the Cubs on July 30 in the second game of a doubleheader against the Brewers, giving up one run on two hits and three walks over six innings. At Iowa, he was 2-2 with a 3.56 ERA in seven starts, striking out 39 over 30 1/3 innings.
Cubs manager Dale Sveum said they want the right-hander to join the team for the final six weeks of the season so pitching coach Chris Bosio can work with him.
The Cubs could've bumped Chris Rusin, but the lefty has done well (2-1, 3.08 ERA) entering Wednesday's start against Cincinnati.
"Not that Villanueva was doing horrible, but Rusin was doing really, really well, and it was hard to bump a guy who's given up one run or no runs every outing since he's been here," Sveum said. "'Villa' has done it before. Losing [Matt] Guerrier, he can be that guy in the sixth or seventh inning when we're winning and get people out."
Guerrier is sidelined with an elbow injury and will not return this season.
Villanueva, 29, began the season in the rotation, but he was shifted to the bullpen when Matt Garza returned from the disabled list. The right-hander is 1-7 with a 4.50 ERA in 15 starts, and 1-1 with a 3.06 ERA as a reliever. Villanueva's only win as a starter came April 18 against the Rangers.
The Cubs' rotation for the upcoming series against the Cardinals will be Arrieta on Friday, Travis Wood on Saturday and Edwin Jackson on Sunday.
Guerrier seeks second opinion on muscle tear
CHICAGO -- Cubs reliever Matt Guerrier is waiting for a second opinion on his right elbow, which forced him out of an Aug. 7 game against the Phillies and ended his season. The Cubs' doctors have told Guerrier he has a tear on his pronator teres muscle of his right elbow, an injury which would require surgery.
"They said the ligament looked good and everything else looked good for my age and the amount of games and innings," Guerrier said Wednesday. "Typically, with someone who is not a pitcher, you could let it go. [The doctor] mentioned that for a pitcher, it's something you definitely want to get fixed, because it will bother you for a while."
The rehab time is projected to be about six to eight months, which is less than the recovery time for Tommy John surgery. Dr. Tim Kremchek, the Reds' orthopedic specialist, will also examine Guerrier's test results.
Guerrier was one of three Cubs injured in the seventh inning against the Phillies. Outfielder Thomas Neal suffered a dislocated right shoulder on a throw, and catcher Dioner Navarro bruised his right ankle in a collision at home plate. Navarro had to be carted off the field, but he is the only one of the three who did not wind up on the disabled list.
"I probably should've just come out of the game, but I felt something similar to it before and thought I could battle through it," Guerrier said. "The way it hurt, it was definitely more than usual, but I didn't know what was going on until after. It was so swollen, I knew something was wrong with it."
The right-hander said he's been dealing with discomfort in his elbow for some time.
"Hopefully I can get it fixed and hopefully get a chance to come back here on a Minor League deal or something next year and have an opportunity to help the team again," he said.
Castro sits as season-long slump resurfaces
CHICAGO -- Starlin Castro did not start Wednesday as manager Dale Sveum decided to give him a breather. It's been a season in which the shortstop has needed a few breaks.
"It's been tough for me this year, because I've never had this [kind of] moment," said Castro, who began this season with a .297 career average but has hit .247 in 2013. "It's always, 'Be good, be good, good, good.' You have these kind of numbers [this year], and you're like, 'What am I doing? What happened?' It's baseball. I'm learning from this. When I get in a slump, I can get out quicker, because I know what happens."
Castro seemed to be back on track in July, batting .292 after hitting .167 in June. But he's 11-for-46 (.239) in his 12 August games, with two doubles, no home runs and one RBI.
The problem? Castro has struggled to hit fastballs.
"You've got to be able to hit the fastball, and if you can't hit the fastball, you'll struggle in this league," Sveum said. "The bottom line is laying off the breaking ball.
"You see guys who aren't having good years, and they're missing the fastball and fouling it off and getting in counts they shouldn't be, and it's because they're missing the fastball."
How can Castro correct it?
"You fix it by understanding hitting and understand the two things connected to the bat are your hands, and you have to use your hands to hit with," Sveum said. "Good fastball hitters use their hands."
Castro has too many moving parts in his swing, and Cubs hitting coach James Rowson has tried to get the shortstop to quiet things down and eliminate some of the movement.
"It's frustrating for him," Sveum said. "The bottom line is obviously performance and being able to get in the box and use your hands there. You can take all the batting practice you want, but you have to be able to do it in the batter's box."
• Prospects Kris Bryant and Dan Vogelbach, both promoted to Class A Daytona, had impressive debuts Tuesday.
Vogelbach, who had been playing at Class A Kane County, homered in the first game of a doubleheader against Clearwater. Pierce Johnson threw 5 2/3 shutout innings, striking out seven in a 5-0 win. Vogelbach, who batted .284 with 17 home runs and 71 RBIs at Kane County, finished with two hits and two RBIs.
Bryant, the second overall pick in June's First-Year Player Draft, homered in the second game, a solo shot in the fourth inning of a 3-2 loss. Bryant batted .354 in 18 games at short-season Class A Boise before getting bumped up to Daytona.
• Josh Vitters may be headed to the disabled list again. The third baseman aggravated his hamstring, which has kept him sidelined much of the season at Triple-A Iowa. Vitters has been limited to 28 games this season because of injuries, and was batting .295. On Monday, in his first game since July 25, he hit a solo home run.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.