CINCINNATI -- Brewers right-hander Shaun Marcum cut short a throwing session Monday because of lingering discomfort in his right elbow, a development that could extend what was supposed to be only a brief stay on the 15-day disabled list.

The Brewers had hoped to have Marcum back on the mound either Saturday or Sunday against the D-backs, after two missed starts. But that plan is now in doubt after Marcum played catch with head athletic trainer Dan Wright in the outfield grass at Great American Ball Park in his first activity since a June 14 start at Kansas City, when he felt tightness in his surgically-repaired elbow.

Marcum told a reporter he was "still sore."

"I'm a little discouraged, too," manager Ron Roenicke said. "We have to do it day by day. If everything goes well, we'll try to come out and throw again [Tuesday], and keep doing it."

Marcum said he'd undergone a series of tests after the start against the Royals, including an MRI scan, that showed no abnormalities. He underwent Tommy John surgery in 2008.

As long as Marcum is sidelined, the Brewers will continue to rely on rookie right-hander Mike Fiers, who has a 2.70 ERA in five starts and one relief appearance. He scattered five hits in 7 1/3 scoreless innings against the White Sox on Sunday.

"Any time somebody comes out and pitches like that, it opens up some eyes," Roenicke said.

Roenicke calls on Brewers to play better

CINCINNATI -- Brewers manager Ron Roenicke had strong words for his team on Monday as it began one of the season's biggest series to date, a three-game set against the National League Central-leading Reds.

It could be a big series, Roenicke said, considering the Brewers entered the week 6 1/2 games back, but only if they begin to play a more fundamentally sound brand of baseball.

"I think it's a way we can get back in this thing, and I've talked about, 'All we need is a good week,' and I still believe that," Roenicke said. "But I'm also concerned, just about the way we're playing. I think that's where my concentration is -- not on who we're playing, but how we're playing."

He added: "I don't see us getting back into it the way we keep playing."

Roenicke was particularly discouraged by a 1-0 loss to the White Sox on Sunday, in which the Brewers made a pair of critical outs at third base, missed opportunities to advance runners and again saw shaky relief outings.

Milwaukee's longest winning streak this season is four games.

"We'll have good games, and then all of a sudden we'll come out and we won't play [well] for a couple of days," Roenicke said. "We have the capability to get back in this."

Estrada eager to rejoin starting ranks

CINCINNATI -- Marco Estrada spent one last day Monday at Camp Hoffman, the afternoon workout for Brewers relievers.

Beginning Tuesday, he'll be a starting pitcher again.

"I'm very happy," Estrada said, no offense intended to his buddies in the relief corps. "I really appreciate the opportunity, because this is what I want to do -- I want to be a starter. I love starting."

The Brewers activated Estrada from the 15-day disabled list after Monday's series opener against the Reds, and he will pitch Tuesday opposite Cincinnati right-hander Bronson Arroyo. It will be Estrada's first outing since May 23, when he strained his right quadriceps legging out a first-inning double.

To make room for Estrada, the Brewers optioned reliever Mike McClendon back to Triple-A Nashville.

Estrada spent his downtime working on some modifications to his windup, which he hopes will improve his command once back on the mound.

"I'm just trying to prepare myself mentally for this," Estrada said. "My first [rehab] outing in [Triple-A] Nashville, I felt a little weird. Not physically, just being out there facing a hitter again. I felt a little out of whack.

"But I'm ready for tomorrow. I'm looking forward to bringing [the adjustment] out and putting it all together."

He threw 75 pitches in his final outing at Nashville and felt "pretty good," so Estrada figures he should be good for 90-100 pitches on Tuesday.

"Physically, I'm there, he said. "It should be a normal outing."

Last call

• One final round of All-Star Game balloting results for National League players will be released on Tuesday, and fans have until 10:59 p.m. CT on Thursday to cast ballots at brewers.com. As an incentive, the Brewers announced that fans who vote at least 21 times for their favorite Milwaukee player will be entered into a drawing to win Ryan Braun's Miller Park Suite for a night, complete with tickets, food and a personal visit from Braun. For details, see the special section of Brewers.com.

Braun is bidding to be voted to the starting lineup for the fifth straight year, and ranked fourth among NL outfielders in last week's balloting update.

• Right-handed pitching prospect Tyler Thornburg, who made a one-start Major League debut last week, is scheduled to make his Triple-A debut on Friday, according to the Nashville Sounds press notes. His Sounds debut was delayed in part because the Brewers are closely monitoring Thornburg's innings this season.

• Another pitching prospect, Drew Gagnon, is getting a promotion from Class A Wisconsin to Class A Advanced Brevard County, though it won't be official for a few days because of a corresponding move.

• With his 0-for-4 output in Monday's 3-1 loss to the Reds, Ryan Braun saw his hit streak come to an end at 18 games.