Howard sustained the injury against Oldham Athletic in the FA Cup fifth round replay 10 days ago and Everton manager David Moyes said he would be out for several weeks.
"He has broken two bones in his back. We don't think he will be out for the season," Moyes told the team's official website.
"We are not sure how long Tim will be out for because, obviously with a goalkeeper, he has to dive around and hit the ground.
"I have let him go away for a few days. It might be that, if everything goes well, he misses next week's game as well and then because of the international break he might be close after the rest because that would be four weeks."
US coach Juergen Klinsmann is likely to call on Aston Villa goalkeeper Brad Guzan for the March 22 qualifier against Costa Rica in Denver, Colorado and the game away to Mexico four days later at the Azteca stadium.
The US lost their opening game in the final qualifying round in the CONCACAF region to Honduras.
Three of the six remaining nations will qualify automatically for the World Cup finals in Brazil in 2014.
Moyes said his goalkeeper only needed time to rest for the injury to become less painful.
"He has two fractures, but they are not bones that are supporting anything," he said. "It is off the spine. They are not weight-bearing. It just needs a bit of healing time.
"It is not like a pure fracture where you need to wait to look at it. It is a question of whether he can cope with the pain. The bones will heal themselves but it's not good."
- Sports & Recreation
- David Moyes
- Oldham Athletic