Last season's NFL rushing champion left practice early Friday after straining his hamstring on a 40-yard touchdown run on the first play of an 11-on-11 scrimmage, the Houston Chronicle reported.
Coach Gary Kubiak says Johnson may return to practice on Sunday, and Foster will definitely miss "a few days." Kubiak is hoping that Cushing returns by the middle of next week.
Foster's injury came during his first practice of training camp. He remained on the sidelines following the incident but declined to speak with reporters.
Foster watched the team's second practice with his leg heavily wrapped, according to the Chronicle.
It was a setback that Kubiak said highlighted the need to have a deep group at the running back position, as players get banged up throughout the season.
"We're strong at that position right now, but y'all know how this works," Kubiak said. "You come out one day or play one preseason game and you go from having a bunch of them to something happening. So let's just keep our fingers crossed that they're all going to be there at the end and we're going to have a good group."
Foster was on Houston's practice squad for much of 2009 before last year's breakout season, when he rushed for 1,616 yards and 16 touchdowns.
The Associated Press contributed to this report.