PITTSBURGH -- Asked last Sunday when top prospect Mitch Keller would join the struggling Pirates’ rotation, general manager Neal Huntington said, “As much as anything else, it’s about opportunity.”

When starter Dario Agrazal was optioned to Triple-A Indianapolis on Tuesday after a rough outing Monday night, the opportunity presented itself. The Pirates are planning to recall Keller in the coming days and plug him into their rotation on Monday in Anaheim, manager Clint Hurdle confirmed before Friday’s series opener at Busch Stadium.

On Thursday, MLB.com confirmed a Pittsburgh Post-Gazette report regarding Keller’s impending promotion, which seemed inevitable when the Bucs sent out Agrazal and pushed up Steven Brault to start Sunday’s series finale against the Cardinals at Busch Stadium.

The Pirates have not announced any plans for their rotation beyond Monday, but Trevor Williams should be in line to start Tuesday’s game in Anaheim -- the second stop on the Pirates’ six-game road trip -- before Chris Archer’s turn comes up again on Wednesday.

This will be Keller’s fourth start in the Majors. He gave up six runs in the first inning of his Major League debut in Cincinnati on May 27, then worked three scoreless innings after that. He returned to make a pair of starts in mid-June and allowed 10 runs (eight earned) on 14 hits and four walks while striking out eight in eight innings.

“Development was not going to happen in that frame, so we got him out, tried to help him recreate some pitch sequencing, the mindset, the prep time, the focus from inning one through whatever -- a closer’s mentality,” Hurdle said.

The Pirates optioned Keller back to Triple-A with a list of objectives specifically focused on his pitch-sequencing and breaking-ball usage. In eight starts, Keller posted a 4.14 ERA but struck out 49 and only walked 10 in 45 2/3 innings. Hurdle said the reports on Keller’s velocity, stuff and pitch usage have all been “very, very positive.”

“With Mitch, it’s just continuing to refine the things that are going to allow him to be successful up here,” Huntington said last Sunday at PNC Park. “You run down the laundry list like you do with every young pitcher. This is the same.”

Keller had been scheduled to start Thursday for Indianapolis, but that outing was pushed up to Wednesday night -- and ultimately rained out before he could throw a pitch. The Triple-A club did not start Keller in either half of its doubleheader on Thursday, so Keller’s most recent start came last Friday. In that outing, he struck out 12 and walked one but allowed four runs in his sixth and final inning.

Around the horn
• All-Star first baseman Josh Bell returned to the starting lineup on Friday, batting cleanup as usual, after a three-day break to work his way out of a lengthy slump. Bell entered Friday batting .209 since June 1 and just .176 with a .529 OPS in the second half.

“I don’t know what to expect. I just know [the extended break strategy has] had some history of success,” Hurdle said. “If nothing else, one of the underlying comments every man has made afterward is that it was really good mentally. It was just really refreshing, mentally, to be in that position.

“It’s a much easier lineup to write with him in it than the one you write for a couple days without him.”

• After starting in left field every day since Corey Dickerson was traded, rookie Bryan Reynolds got the nod in right field on Friday night. Jose Osuna started in left field for the first time since July 29 and just the second time this season.

Hurdle said Osuna would be an option to start in left occasionally at PNC Park, but the Pirates would prefer to let Reynolds -- who has more range than Osuna -- handle their home park’s more spacious confines and the tricky notch in left-center.

• This series is a homecoming of sorts for right-hander Geoff Hartlieb. The rookie reliever was born in Highland, Ill., which is about 30 miles northeast of St. Louis, and he was drafted out of Lindenwood University, which is about 25 miles northwest of Busch Stadium.