NASA Administrator Charles Bolden made the remarks at NASA's Marshall Space Flight Center in Huntsville, Ala., while addressing NASA employees last week, SPACE.com reported Monday.
"We are hoping to fly a third shuttle mission in June, what everybody calls the launch-on-need mission ... and that's really needed to [reduce] the risk for the development time for commercial cargo," Bolden said.
"If there's any delay in ... delivery of commercial capability to take cargo to station, we could find ourselves in a situation as bad as having to de-man the station or take it down to three people, and we really don't want to do that," Bolden said.
NASA intends to rely on commercially operated space transportation systems to deliver cargo and eventually astronauts to the space station after it retires its shuttle fleet next year.
But development of privately owned rockets and spacecraft designed to ferry cargo to the orbiting outpost is taking longer than expected.
The commercial development of cargo delivery systems is being carried out under NASA's Commercial Orbital Transportation Services program.