The 25-year-old was injured during his side's 2-0 home defeat by St Johnstone in the Scottish Premiership.
Manager Jim McIntyre told BBC Scotland: "We will know more about Liam's injury later on.
"But we think it is a medial ligament injury and we are hoping he's not out too long."
Boyce, who has won seven caps for his country, is County's top scorer this season with six goals this season - all in his last six games.
And he was named in Michael O'Neill's squad on Thursday for the Group C double header on 8 and 11 October.
But he had to be helped off the field five minutes before the end of County's match on Saturday.
Boyce collided with Zander Clark as the goalkeeper parried a shot from Chris Burke, the former Scotland winger making his debut for County.
Northern Ireland are already without Will Grigg, with the Wigan Athletic striker having been left out for family reasons.
Ross County striker Liam Boyce is an almost certain withdrawal from the Northern Ireland squad for World Cup qualifiers with San Marino and Germany.