Toronto FC will be without veteran defender Drew Moor for Saturday's match against Columbus Crew SC.
The MLS team says a medical exam following training this week showed a cardiac arrhythmia – or irregular heartbeat.
The team said in a statement that further testing is needed "to determine diagnosis and management."
Moor will not travel with the to Columbus "as a precaution."
The 33-year-old Texan has anchored the Toronto defence since joining the club prior to the 2016 season as a free agent from Colorado.