The former Tottenham ace was subbed off just after the hour-mark of TFC's 3-0 loss at Real Salt Lake on the weekend with a hamstring problem.
Any absence of Defoe would be a major blow for the Reds especially seeing as he has scored all three of the Canadian clubs goals so far this season but Nelsen was not in crisis mode.
"We'll just have to wait for the scans to come back," said Nelsen. "Hopefully it's clear."
Defoe missed Tuesday's training session as Toronto prepare for the weekend's meeting with Eastern Conference leaders Columbus Crew.
Since his shift from the Premier League, the England international scored a double on debut as TFC beat Seattle Sounders 2-1 on March 15 before netting the winner in the 1-0 triumph over DC United on March 22.
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