CHICAGO – Left-handed starting pitchers have been the Cubs’ kryptonite all season.

The struggles confound manager Dale Sveum because he expected the offense to thrive off the lefties and continue last year’s success. The Cubs hit .263 versus lefties, fourth-best in the National League, and their 2010-11 combined .287 batting average led the majors in that span.

Sveum had good reason to believe countering a lefty with a right-handed stacked lineup would produce the same results and be a strong point this season for the Cubs. Sveum stuck with his normal righty lineup Sunday against Boston Red Sox southpaw Franklin Morales. Sveum could reconsider his atypical lineup and let lefties Bryan LaHair and David DeJesus start but he said they haven't been consistent against left-handers either.

“It’s almost a fluke,” Sveum said. “ … It’s not that they’re doing horrible, we’re just not getting the slugging percentage at all against left-handed pitching, which is holding us back from scoring multiple runs in an inning.”

Unbalanced interleague schedule: The Cubs’ haven’t had many easy matchups in interleague play this season.

They concluded a three-game set against Boston on Sunday right after a tough series versus the Tigers in front of a raucous Detroit crowd at Wrigley Field. Add in six games this season against their crosstown rival White Sox and three in Minnesota and the schedule certainly has challenged the Cubs, who are 3-9 in interleague play this season.

When asked if baseball needs to find a way to equalize matchups across the majors, Sveum was unsure.

“That’s the million-dollar question with interleague play because year to year teams can be a lot better,” Sveum said. “I think the one thing that hurts it a little bit is teams having to play teams six times. It’s great, don’t get me wrong, but I think that’s where the balance gets off a little bit.”

DH in play: The Cubs gain a designated hitter when their three-game series against the White Sox begins today at U.S. Cellular Field, and outfielder Alfonso Soriano again will fill the role, Sveum said.

Soriano was the Cubs’ DH for their series in Minnesota and he thrived in the role. He hit .462 with three home runs and six RBIs in three games. With the White Sox starting three right-handers – Zach Stewart, Jake Peavy and Gavin Floyd – Reed Johnson likely will get the starts in left field, Sveum said.