Gustavsson was taken to the hospital as a precaution with an elevated heart rate following the first period of Toronto's 3-0 victory over the Montreal Canadiens on Tuesday night.
Gustavsson, who made eight saves, informed the team of his condition at the end of the opening period. The Swedish rookie had minor heart surgery during training camp after he experienced a racing heartbeat.
Maple Leafs coach Ron Wilson said Gustavsson's heart rate went up during a Montreal power play early in the first and "came down on its own in between periods." But the decision to have him examined further was firm.
"The last incident that he had and then had the ablation, his heart never came down until he got to the hospital and they gave him some medication to slow his heart down," said Wilson, who added that Gustavsson would spend the night in the hospital and be examined by his cardiologist in Toronto on Wednesday. "We're not sure what this event was, so let the doctors decide that."
Toronto general manager Brian Burke said Gustavsson is expected to be fine.
"We're just being ultra-cautious," Burke said.
"As soon as I heard that, when the trainer came, I already got Joey going no matter what his heart was going to do because I'm not going to be responsible for someone keeling over during a game," Wilson said. "A game is not that important."
"We were up 2-0 and you knew they were going to come out and throw everything at the net, especially when they saw me in there," MacDonald said.
Carey Price stopped 28 shots in his ninth straight start for Montreal, which has lost three straight.
"It was a difficult night," coach Jacques Martin said. "Sometimes you have a few guys who aren't playing to their potential, but tonight we had trouble finding one who was."
The Canadiens, who play in Buffalo on Thursday, will celebrate their 100th anniversary Friday when they host the Boston Bruins.
Scott Gomez returned for Montreal after missing four games with a groin injury.
Orr opened the scoring with his first goal in 26 games with Toronto. The Maple Leafs' enforcer took a pass from Tomas Kaberle and beat Price with a wrist shot 9:24 in to the dismay of the sold-out crowd of 21,273.
Kessel, who was injured and missed the first two meetings between the longtime rivals in October, got his first assist at 12:51 when Finger drove a slap shot home to increase the lead to 2-0.
Stajan added the Maple Leafs' third goal on a breakaway 6:53 into the second. Kessel made a long pass up the middle to Stajan, who had slipped behind Montreal defensemen Roman Hamrlik and Jaroslav Spacek.
The Toronto center had enough time to settle a bouncing puck before driving in to put a shot into the top right corner for his eighth goal.
"Stajan basically just cheated up," Spacek said. "He didn't stay with me at all, he just cheated up and Kessel saw him wide open behind us. It's tough to chase someone if he's 40 feet ahead of you."
The sale of the Canadiens to a group led by the Molson family was approved earlier in the day by the NHL board of governors. ... Gustavsson started games on consecutive nights for the second time this season. He made 28 saves against the Sabres on Monday night. ... MacDonald got his only career shutout with the New York Islanders on March 27 in a 2-0 win in Detroit. ... Canadiens RW Matt D'Agostini also returned after missing 13 games because of a concussion and a knee injury.