CHICAGO -- Cubs center fielder Ian Happ stumbled a bit before catching William Fowler's fly ball to open the Cardinals' first inning. Right fielder Jason Heyward was there to back up the rookie and razz him a little.

"I said, 'You've got two left feet,'" Heyward said of the good-natured joking.

"My main goal is to catch every one of them," Happ said. "It may be ugly, but as long as I catch it, [it's OK]."

The rookie's other goal is to contribute to the Cubs offensively, and he did just that Sunday night, hitting a pair of home runs in a 7-6 victory over the Cardinals. It was Happ's first career multi-homer game and he finished with four RBIs, also a high.

"He's going to continue to do a better job of knowing what to not swing at," Cubs manager Joe Maddon said of Happ. "He doesn't have to do mechanical adjustments, he doesn't have to spend 1,000 hours in the batting cage. It's a focus thing. He has a good idea of the strike zone. When he really refines it here, heads up."

Happ hit a solo shot in the third that stayed just fair, and a three-run homer in the fourth. The Cubs' first-round pick of the 2015 Draft, Happ had hit two home runs in his first three big league games, but was batting .114 in his last 11 games with one extra-base hit prior to Sunday.

"It's a process," Happ said. "You're going to go through ups and downs. It's a long season, but I've felt good about where my swing is."

So far, 10 of Happ's 16 hits have been for extra bases. He's the first Cubs leadoff hitter to record a multi-homer game since Matthew Szczur did so last Aug. 12 against the Cardinals.

"He does what he can to help us, and that's what I feel like everybody's done a great job of," Heyward said. "He handles it like a professional, he's the same guy every day."

Happ did laugh at Heyward's joke about his footwork. Having a four-time Gold Glove Award winner playing next to you has helped Happ progress defensively.

"His baseball IQ is unbelievable," Happ said of Heyward. "I can't say enough about how much I've learned from him."

In his first eight games in the big leagues, Happ batted .357 with two home runs, four doubles, one triple and five RBIs. The rookie never thought his struggles would prompt the Cubs to send him back to the Minor Leagues.

"No, I'm going one day at a time and trying to help the club every day," Happ said.