Backe left in the second inning of the Astros' 5-3 loss to San Francisco on Thursday. He flew to Houston and had the elbow examined on Friday.
Right hander Ezequiel Astacio, who started 14 games last season, was recalled from Triple-A Round Rock. Astacio went 3-6 with a 5.67 ERA in 22 appearances in 2006.
Backe skipped his first scheduled start of the season on April 5 -- his 28th birthday -- because of a sore back. He returned two days later and earned a win against Washington, allowing a run on three hits in six innings of work.
The right-handed Backe, who went 10-8 in 26 appearances in 2005, fills a pivotal role for the Astros behind ace starters Roy Oswalt and Andy Pettitte. Last year, Houston had Roger Clemens, but the seven-time Cy Young winner is now a free agent pondering retirement and cannot re-sign with the Astros until May 1.
The 23-year-old Nieve will make his first major-league start on Sunday in Arizona and the 24-year-old Buchholz, who relieved Backe on Tuesday, will make his first major-league start on Monday against Milwaukee.