Steyn recently returned to action from a shin/fibia injury at the end of last year and has been in fine form he picked up a knee knock in Friday night's Champions Cup win over Racing 92.
The injury has ruled Steyn out of the Murrayfield opener against Italy in February but Townsend hasn't ruled out the prospect of the winger returning later in the tournament.
Townsend told Scotland Rugby News: "Kyle's not in the squad because he can't train the next two weeks. But he'll be in our group.
"We'll know more next week on how that knee is going to be. He's not going to be available for Italy but, if he does come back into consideration, maybe for the Ireland game, maybe England will be probably the more realistic focus.
"But a lot will depend on how it's settled down over the next few days. It's really disappointing for Kyle and for us because he was in great form.
"He obviously missed out in November but got back into brilliant form over the last few weeks and we hope he's still got a big part to play in the Six Nations."