Gustavsson's return Wednesday came a day after he was taken to the hospital with an elevated heart rate following the first period of Toronto's 3-0 victory over the Montreal Canadiens.
Toronto took the precautionary move Tuesday night because the Swedish rookie needed minor heart surgery during training camp following a similar incident.
Toronto general manager Brian Burke said the Leafs are "just being ultra-cautious."
Gustavsson was examined at a Montreal hospital on Tuesday night and released.
The team said it would update his condition on Thursday.