"We want someone that can do a little bit of everything," O'Brien said.
The Texans had running backs Pierre Thomas, Joe McKnight and Ben Malena in to work out Wednesday.
Thomas' agent, Lamont Smith, tweeted Wednesday that his client and the Texans couldn't agree to a deal.
#NFL Pierre Thomas had a great visit and work out with texans, unfortunately we were unable to agree on terms.— Lamont Smith (@CLamontSmith) August 5, 2015
Foster is still being evaluated for a groin injury he suffered during Monday night's practice, O'Brien said Wednesday. O'Brien said it is still not clear how much time Foster will miss.
According to sources, Foster is likely to miss part of the regular season and need surgery to repair the injury.
"It's heartbreaking for me as a friend. As a teammate he's a very instrumental part of our team," Texans left tackle Duane Brown said. "... We still don't know how long he'll be out but we have a lot of guys who can step up."
In Foster's absence, second-year back Alfred Blue takes over first-team duties. Blue was drafted in the sixth round last season.
Jonathan Grimes, entering his fourth season in the NFL, and fullback Jay Prosch also return from last season. The Texans also added Chris Polk in free agency and drafted Kenny Hilliard out of LSU in the seventh round this year.