SCOTTSDALE, Ariz. -- The Rockies entered Sunday's tilt with the A's averaging seven runs a game, the kind of offense they're counting on as a young pitching rotation continues to mature after starters posted a 5.81 ERA last season and won only 29 games.

So when top prospect Nolan Arenado belted his third Cactus League dinger Sunday in the second inning -- a towering solo shot to left -- and drove in both runs from a lineup filled to the brim with seven likely Opening Day starters, the buzz only grew about the 21-year-old's potential and the contributions he could make with Colorado.

Arenado has played a season each in Casper (Rookie), Modesto (Class A), Ashville (High A) and Tulsa (Double-A), and he is slated to play at Triple-A Colorado Springs this year after Chris Nelson solidified his hold on third base with a .301 campaign in 2012, knocking nine homers and 53 RBIs in 111 games for the Rockies.

"You don't want to ever put a limit on a kid like that and say he can't win a job," manager Walt Weiss said Sunday. "He's made a good impression. Chris Nelson had a very good second half last year, too."

Nelson hit .345 from Aug. 1 on, best among all Major League third basemen, including Triple Crown-winner Miguel Cabrera's .344.

"We got another month to go, and the picture will get a little clearer with some more games under our belt," Weiss said. "But Nolan's certainly made a very good impression."

Results mixed for Rockies pitchers vs. A's

SCOTTSDALE, Ariz. -- Early in spring and nearly two years after Tommy John surgery on his left elbow, Colorado's Jorge De La Rosa is still struggling to find his form.

De La Rosa was on the disabled list from May 25, 2011, through Sept. 20, 2012, and he made three starts upon his return last season, posting a 9.28 ERA in 10 2/3 innings. In two Cactus League starts so far this spring, the 31-year-old southpaw has a 7.36 ERA in 3 2/3 innings, capped by a rough outing Sunday when he faced 11 batters in 1 2/3 innings. He allowed two hits while walking three and hitting a batter to force in a run in a 7-2 loss to the A's.

"We'll have some patience with Jorge, because he hasn't been out there much in the last year and a half or so," manager Walt Weiss said. "He's a guy we're counting on, and he knows that. We got another month here, so we're not going to get too carried away. It wasn't real pretty, but he's got some time to figure some things out."

De La Rosa has won 39 games for the Rockies in five seasons, three cut short by injury, and Colorado is counting on him to be a top-of-the-rotation pitcher in 2013.

At the other end of the rotation, Tyler Chatwood is vying for a fifth starter's slot, and his outing epitomized a mixed bag following De La Rosa for three innings Sunday. Chatwood faced nine batters in his first inning, loading the bases with a walk and two singles, then forcing three runs across the plate with two hit batters and a walk. As he noted, he was ahead in the count with "all the guys I hit," but any time three runs score as a result of four free passes, something is off.

Chatwood adjusted beautifully, however, retiring seven in a row and striking out five on the day, demonstrating a resiliency essential for Rockies pitchers throwing half their innings at Coors Field.

"As a starter, if you have a bad inning, you can't really dwell on it," Chatwood said. "You have to go back out there for five or six more innings. It's something to build off and try to eliminate those long innings."

The determination was not lost on Weiss.

"It was nice to see him battle back," the skipper said of Chatwood. "The first inning looked like he wasn't going to be able to give us three, but he battled back and was able to do that. That's part of who he is. He'll go out there and grind and do whatever he needs to compete.

"That's got to be our identity: to compete, regardless of the circumstances."

Rockies acquire Cassevah from Angels

SCOTTSDALE, Ariz. -- The Rockies expanded their big league Spring Training roster to 66 Saturday when they signed right-handed reliever Bobby Cassevah to a Minor League contract and invited him to camp.

Cassevah had been outrighted by the Angels on Friday, but he declined a Triple-A assignment and elected free agency instead. The Rockies scooped him up a day later to contend for their 'pen.

Though he pitched in only four games for the Angels in 2012, Cassevah made 50 appearances for the club since his big league debut in April of 2010. He owns a 3.20 ERA in 64 2/3 innings pitched, with 34 strikeouts against 33 walks. Perhaps most appealing to the Rockies, he has a 65.9 percent groundball ratio.

Pitchers and catchers reported to the Rockies Minor League camp Saturday, with position players reporting Friday. Cassevah can expect at least an appearance or two in Cactus League play as his new club gets a close look at him. He made one spring appearance with the Angels on Feb. 24, throwing one scoreless inning while allowing a hit and two walks.