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8/31/2013 8:33 P.M. ET

Oswalt to make rehab start at Grand Junction

DENVER -- Opposing batters may be a bit starstruck when Roy Oswalt steps to the mound in Grand Junction on Tuesday night.

Oswalt said he planned to make a rehab start with the Rockies' Rookie affiliate while working back from a left hamstring injury sustained July 7. He threw 50 pitches Friday in his third simulated game and said he had no issues.

Manager Walt Weiss is hoping the right-handed Oswalt will need just one rehab start before rejoining the Rockies' rotation but called that the "best-case scenario."

Oswalt, who turned 36 Thursday, struggled in four starts with the Rockies, but Weiss still sees potential in the former Astros ace.

"We want to get another look at Roy, see if there's a fit even beyond this year, and if he can perform like we think he can, he's going to be a fit for us," Weiss said. "So it's nice to have that veteran guy around that brings a lot of wisdom, a lot of competitiveness. It would be good to get a look at him here down the stretch."

Oswalt lasted through the sixth inning just once in those four starts and allowed four runs or more in every start except his final one, which lasted just 1 2/3 innings because of his injury. He is 0-4 with a 7.64 ERA, numbers that are slightly worse than those he posted last season with the Rangers.

"There's a lot of things we like about him," Weiss said. "I still think he can perform. I know his numbers aren't what he would want them to be, but I think there's still a lot left."

Rosario giving Rox offense from behind plate

DENVER -- Walt Weiss can see Wilin Rosario developing into a more intelligent hitter, sitting on more pitches and understanding the pitching sequences.

And it is paying off big time.

Lost in Todd Helton's memorable performance in Friday 9-6 victory was a stellar night from Rosario, who went 4-for-5 with a double and scored two runs. The four hits tied a career high set in April.

Rosario is nearing the end of his best offensive month as a big league hitter -- batting .356 with five homers, five doubles, 15 runs and 20 RBIs (a career high) before ending August on Saturday against the Reds. With a month to play, he has already surpassed his doubles, hits and RBI totals from his rookie campaign.

His 72 RBIs are a season record for a Rockies catcher, and his 203 total bases are tied with Yadier Molina for second most among NL catchers, trailing only the Giants' Buster Posey.

"I said it this spring; I felt like this guy's going to be not only a guy that hits for a lot of power but a good hitter," Weiss said. "Had some tough spots this year, where he was getting pitched to a certain way and maybe struggled with the adjustments for a certain time. But he's figuring some things out. We're seeing the results of that."

Rosario's impressive offensive numbers allowed him to finish among the top five for National League Rookie of the Year in 2012, but his defense was a major liability. He allowed 21 passed balls and committed 13 errors, numbers that are down to nine and eight, respectively, in Year 2.

"He took a lot of criticism," Weiss said. "He got thrown into the fire; he struggled and still maintained that positive outlook and still showed up every day. ... When a guy gets beat up by the game and he's still able to maintain confidence and the type of outlook he has, that says a lot to me about him as a player."

At 24, Rosario is just beginning to tap into his offensive potential. He is the rare man behind the plate who can hit for power and average.

"When you look at the production, he's got a chance to be a very elite player," Weiss said. "A guy at that position, that gives you that type of offense, it's a rarity."

Fowler better, but hopes of quick return fade

DENVER -- Dexter Fowler said Saturday his knee was feeling better but had not improved enough for him to make it back into the starting lineup.

Fowler missed his fourth straight start Saturday against the Reds with a left knee bruise sustained in a win over the Giants on Monday.

"He made a little bit of an improvement from yesterday, but still day to day," Rockies manager Walt Weiss said. "Still, I would imagine a couple days away."

As he has in three of the four games Fowler has missed, Charlie Blackmon played center. Corey Dickerson, who has become a more regular piece of the lineup with Carlos Gonzalez on the disabled list, hit leadoff for the third straight game.

Between July 27, his second callup, and Friday, Dickerson was hitting .300 with two homers, six RBIs, five doubles and three triples.

Worth noting

• Only one or two players may join the Rockies on Sunday when the roster expands for September call-ups, Weiss said, adding that more players would join the big league clubhouse once Triple-A Colorado Springs' season ends Monday and that a player or two from Double-A Tulsa was in contention for a roster spot.

Ian McCue is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.