Just 18 games into the season, the Astros’ elevator to Class AAA Oklahoma City is already running strong. Pitchers Dallas Keuchel, Edgar Gonzalez, Brett Oberholtzer and Paul Clemens have been called up. Keuchel also has been sent back down, while Xavier Cedeno and Gonzalez have been designated for assignment. Reliever Travis Blackley was acquired in an early season trade. Rule 5 pick Josh Fields was placed on the disabled list. Throw the names together, add in Astros starting pitchers who often have struggled to reach six innings and a team that posts an MLB-worst 5.40 ERA, and the organization has quietly introduced tandem pitching units to the big leagues during the start of the 2013 campaign.

Not officially. Only the Astros’ full-season minor league clubs are experimenting with “piggyback” tandems. But with big league starters Erik Bedard, Bud Norris and Philip Humber allowing a combined 14 hits, 20 earned runs and eight walks in just 11⁄3 innings last week and the Astros unwilling to rush their premier young arms to the majors, manager Bo Porter acknowledged his struggling 5-13 team would be in even worse shape without being able to roll out former minor league starters in extended relief.

“Having guys down in the bullpen as long guys is vital to the overall success of the team,” Porter said prior to the Astros’ 5-4 loss to the Cleveland Indians on Sunday at Minute Maid Park. “You look at the short sample we’ve had this year, they have saved us quite a few times.”

Through 16 combined minor league seasons, Oberholtzer, Clemens and Keuchel amassed 317 games, with 80.1 percent of their appearances coming via starts (254). Everyone in the young trio has yet to start a game for the 2013 Astros, though, despite racking up 251⁄3 innings in eight appearances. Clemens, who made his MLB debut last Tuesday, has more innings (122⁄3) than 34-year-old starting lefthander Erik Bedard (112⁄3), who was held to just four innings and 75 pitches in Sunday’s 5-4 loss to the Indians because of his injury history.