ST. LOUIS — The need for bullpen help, the Royals decided, outweighed any desired extra bench flexibility Friday when they embarked on a run of six straight interleague road games.

“I’ve fried the pen,” manager Ned Yost said before ticking off problems before the series opener against the St. Louis Cardinals at Busch Stadium. “(Greg) Holland is probably unavailable tonight. (Aaron) Crow is iffy. I’d like to give him one more day.

“(Kelvin) Herrera is unavailable tonight. (Tim) Collins and (José) Mijares should be unavailable tonight. I’ve had to bang all of these guys; they’ve all pitched two days in a row. I’ve got to keep the arm. It’s simple.”

The Royals sought to address the problem prior to Thursday’s game against Milwaukee by adding an eighth reliever; they summoned veteran right-hander Roman Colon from Class AAA Omaha. At the time, Yost said the promotion might last only a day.

The reason: The Royals wanted to ponder lengthening their bench because of the extra pinch-hitting situations that routinely occur in games played under National League rules without the designated hitter.

Yost opted for the arm — and called on Colon to pitch the seventh when a stiffening calf muscle forced starter Vin Mazzaro to depart with a 3-0 lead. Colon gave up three hits and two runs and failed to get out of the inning.

The Royals called on Mijares to get the final out in the seventh before turning to Crow in the eighth. Jonathan Broxton worked the 3-2 victory by surviving a wild finish in the ninth.

“My mind-set was I had Crow for the eighth inning and Broxton for the ninth,” Yost said. “So the seventh inning was pivotal. We had to get through that seventh inning to get to Crow in the eighth.”

They did…barely.

The Royals entered the weekend leading both leagues with 2352/3 relief innings. Colorado ranked second at 2211/3, followed by Minnesota at 2202/3.

And trend is growing.

Yost’s bullpen corps has now chewed up 13 2/3 innings over the last four days while and has combined for 32 innings in the last eight games.

Hosmer plays; Butler sits

Butler went to the bench Friday when Yost opted to start Eric Hosmer at first base rather than shuffle his defensive alignment to keep both in the lineup. Hosmer played right field for two games last weekend in Pittsburgh.

Why Hosmer over Butler?

“Better defense,” Yost explained, “and it gives us a lefty against righty (Cardinals starter Kyle Lohse). And Hos is swinging the bat now.”

Hosmer was batting .320 over his previous 21 games after bottoming out at .172 by going zero for three on May 20.

“The only upside,” Yost said, “is I’ve got a guy (Butler) who I can bring off the bench in an RBI situation that I feel good about. Trust me, I can hardly wait to get through these six games and get back to our game.

“It was so nice after those three games in Pittsburgh to get back to (Kauffman Stadium) with our team and our lineup the way it was built to be. I mean, this is not the way we built our roster or our team — to play National League rules. But we have to do it.”

Walk-off reprise

The Royals didn’t have a walk-off victory in their first 29 home games before pulling the trick Wednesday and Thursday against Milwaukee. At this point last year, they already had nine walk-off victories. They finished the season with 12.

Minor details

Class AAA Omaha outfielder Wil Myers made another appearance on Baseball America’s weekly Prospect Hot Sheet, which seeks to identify which of the game’s top prospects are currently excelling.

Myers, 21, went nine for 28 with two homers over the last week and is batting .321 with 10 homers and 30 RBIs in 28 games since his promotion from Class AA Northwest Arkansas. Overall, he is batting .333 with 23 homers and 60 RBIs in 63 games.

Top prospects assigned

Outfielder Bubba Starling is officially heading to short-season Burlington (N.C.) in the Appalachian League, and right-hander Kyle Zimmer will open his pro career at Surprise in the Arizona Rookie League.

Those are two of the roster assignments announced by the Royals in staffing their three short-season clubs.

Here are the rosters.

Idaho Falls (Pioneer League): pitchers Kevin Allen, Bryan Brickhouse, Fernando Cruz, Cory Hall, Chase Hentges, John Killen, Joseph Lopez, David Middendorf, Alec Mills, Spencer Patton, Clayton Schulz, Sam Selman, Kyle Smith, Zeb Sneed, Matt Strahm, Andrew Triggs and Ali Williams; catchers Travis Lane, Parker Morin and Jin-Ho Shin; infielders Rainier Bello, Nick Cuckovich, Nick DelGuidice, Carlos Garcia, Jack Lopez, Adrian Martinez, Adalberto Mondesi and Jared Schlehuber; outfielders Julio Aparicio, Jerico Blanco, Ethan Chapman, Elier Hernandez and Ysmelin Alcantara.

Burlington (Appalachian League): pitchers Parker Bangs, Christian Binford, Yender Caramo, Patrick Conroy, Brian Edelen, Daniel Hernandez, Jakob Junis, Josiel Martinez, Julio Morales, Mark Peterson, Lincoln Rassi, Colin Rodgers, Freddy Rodriguez, Andrew Stueve, Dan Stumpf and John Walter; catchers Cam Gallagher, Beau Maggi and Alexander Marquez; infielders Humberto Arteaga, Patrick Leonard, Adrian Morales, Henry Moreno, José Rodriguez and Mark Threlkeld; outfielders Chris Elder, Fred Ford, Terence Gore and Bubba Starling,

Surprise (Arizona Rookie League): pitchers José Brazoban, Darwin Castillo, Torey Deshazier, Andrew Durden, Ashton Goudeau, Cruz Guevara, Shane Halley, Hunter Haynes, Zach Lovvorn, Stephen Lumpkins, Jake Newberry, Aroni Nina, Cesar Ogando, José Rodriguez, Dylan Sons, Matt Tenuta, Christian Witt and Kyle Zimmer; catchers Pedro Gonzalez, Chad Johnson and Luis Villegas; infielders Mark Donato, Diego Goris, Ryan Meade, Alfredo Patino, Mauricio Ramos, Ramon Torres and Lewis Urena; outfielders Jerrell Allen, Cameron Conner, Ariel Estades, Alexis Rivera, Lance Harper and Alex Hudak.

Looking back

It was 33 years ago Friday — June 15, 1979 — that the Royals mounted their biggest comeback victory in history by rallying to win 14-11 at Milwaukee after trailing 11-2 after four innings.

The Royals capped the comeback with an eight-run ninth inning that Willie Wilson punctuated with a three-run homer against Bill Castro. In doing so, Wilson became the first Royal to hit homers from both sides of the plate in the same game.

Etc.

• The Cardinals still own a 38-29 advantage in the I-70 Series, although the Royals are now even at 14-14 St. Louis. (No, those numbers don’t count the 1985 World Series.)

• The Royals have played nine straight games decided by one or two runs. They were 5-4 in that span.

• Alcides Escobar had not scored a run since May 30 before scoring on Jarrod Dyson’s single in the second inning. Dyson’s RBI was his first since May 19.

• When Humberto Quintero struck out as a pinch-hitter in the eighth inning, it dropped the Royals to a collective three-for-25 this season in pinch-hitting attempts.

• Vin Mazzaro beat a National League team after losing all six previous decisions. He also lowered his ERA to 2.57 by pitching six scoreless innings for the second time in three starts.