The 21-year-old had to be helped off the pitch in the 38th minute during his country's 2-1 friendly win over Norway on Friday night and will be given a scan to diagnose the severity of the problem.
Olsen fears Eriksen could be sidelined until the end of the year, which would rule him out of Tottenham's key clashes against Manchester City, Manchester United and Liverpool.
“We don't know if he's out for four or six weeks or how long it will take,” the Dane told reporters. “It's bad for him at a new club.”
Spurs have struggled to score goals this season, notching just nine times in 11 games, and the injury to Eriksen will be a huge blow for Andre Villas-Boas, who saw the Dane create a season-high nine chances against Newcastle.