CHICAGO -- Cubs pitcher Carlos Marmol and catcher Steve Clevenger will begin rehab assignments this week at Triple-A Iowa, but neither are expected to rejoin the big league team until after next weekend.

Marmol, sidelined since May 12 with a strained right hamstring, will throw on the side Monday for Iowa, then throw a simulated game Wednesday, and, if all goes well, he'll get into a Minor League game Friday.

Clevenger, out since April 26 with a strained right oblique and rib cage injury, has been taking batting practice this weekend for the first time since he was injured. He will play Tuesday and ease his way into game action, gradually working up to nine innings as the week progresses.

Welington Castillo has a mild sprain of the MCL in his right knee, and is expected to miss four-to-seven days, depending on how he responds to treatment. He was injured on Friday on the last pitch Kerry Wood threw.

"I felt like I couldn't bend my knee or go side to side," Castillo said of the injury.

He hopes to resume baseball activities on Wednesday.

Reliever Blake Parker, who came out of Saturday's game, underwent an MRI on Sunday which showed he has a mild right hamstring strain. Parker is listed as day to day.

Rizzo impressing with Triple-A Iowa

CHICAGO -- The Cubs' offense has struggled this season, but one player who hasn't is Anthony Rizzo, who is batting .353 in 40 games at Triple-A Iowa. Could the first baseman join the big league team soon?

"That's definitely going to be talked about," Cubs manager Dale Sveum said on Sunday. "He's done everything he can down there, but once again, when you bring somebody up like that, he has to play every day. That's the million-dollar question. How do we get that playing time?"

Rizzo, the Cubs' No. 1 prospect, is considered a potential Gold Glove first baseman, and Bryan LaHair has done fine at first for the Cubs. LaHair could move to one of the outfield corners to make room.

One possibility would be to add Rizzo when the Cubs play Interleague games in American League ballparks and need a designated hitter. They travel to Minnesota June 8-10 and play the White Sox at U.S. Cellular Field on June 18-20.

"It's definitely something we'll talk about, and probably around the beginning of June and Interleague Play, when you need a DH," Sveum said.

The problem is, when the Cubs call Rizzo up, it's not going to be a short fix.

"I think he's a huge asset at first base," Sveum said of the 22-year-old Rizzo. "A left-handed first baseman, and he's really, really good over there. I don't think we'd move him to the outfield or move him around at all."

Rizzo has made six errors at Iowa, but he's also hit 14 home runs, totaled 39 RBIs and has a .706 slugging percentage. He was promoted to the big leagues last June with the Padres after hitting .365 at Triple-A Tucson, and struggled, batting .143 in 35 games. Rizzo was sent back to Tucson then recalled in September, and he hit .133 in 14 games. The Cubs acquired Rizzo in a January trade with San Diego.

The Cubs rank 10th in the National League with a .246 team batting average and are 12th with 29 home runs entering Sunday. They need some pop.

"Our offense has to step up, there's no getting around it," Sveum said. "We have to step up to get three-run innings, multiple crooked numbers all the time, as much as you possibly can. It's too difficult to not do anything off starting pitchers and think you're going to do stuff off the back end of bullpens."

Soriano producing despite pain in knee

CHICAGO -- His left knee hurts and it's tough to run, but it hasn't stopped Alfonso Soriano, who was batting .310 this month entering Sunday.

His knee is most uncomfortable when the weather is cold, Soriano said on Sunday. Now that it's warming up, he's feeling better. Still, you can tell he's not right when he runs.

"This year, it's bothered me a little bit," the Cubs' left fielder said Sunday. "But I like to play, so I don't want to sit down and watch the game. It's pain, but I'm 36 years old and I have to learn to play with pain. I'm not 25 any more. I love to play and I want to help the team win."

Soriano had three hits on Saturday, including a two-run home run in the ninth inning of a 7-4 loss to the White Sox.

"He's in one of those streaks where he's feeling pretty good at the plate and seeing the ball good and putting some good swings on the ball," Cubs manager Dale Sveum said.

After hitting .237 with no home runs in April, Soriano has three homers in his last five games.

"He's the biggest surprise, as far as me coming from the other side of the fence, as far as his work ethic and the way he goes about his business and how much he wants to play every single day with the pain he has to go through with his knees," Sveum said. "He's a very refreshing guy to be around all the time."

Soriano's also been one of the few bright spots lately.

"It's very tough because you know I believe in this team, but this is the game, it's up and down," Soriano said of the Cubs' recent struggles. "The last five games, we haven't played good, but maybe this week, we start playing better. I believe in this team, and think, sooner or later, it's going to change."

There has been a lot of roster turnover because of injuries, with the Cubs making 10 moves in the last four days.

"Those kids, they're in the big leagues for one reason," Soriano said. "Nobody's in the big leagues because they're cute; it's for one reason -- they have the talent. I believe in those guys and we'll be fine."

Extra bases

• Jeff Samardzija talked to Paul Konerko on Saturday, one day after the pitcher hit the White Sox first baseman in the head with an errant pitch.

Samardzija said Konerko told him he was leaning in more because of the previous pitch, and that the first baseman couldn't get out of the way of the splitter. Both understood what happened was an accident.

• The Cubs' rotation will stay the same with the upcoming off-day on Thursday, so Ryan Dempster will open the series at Pittsburgh on Friday, trying to end a 16-start winless streak. Travis Wood will be called up Tuesday to face the Astros and take Chris Volstad's spot in the rotation. Volstad was optioned to Triple-A Iowa on Friday

• The Cubs have made 10 moves on the 25-man roster in four days, starting Wednesday when pitcher Lendy Castillo was placed on the disabled list. The list includes the retirement of pitcher Kerry Wood on Friday.

"We're running out of lockers," Dempster said. "That's probably the biggest thing."