Gustavsson made eight saves before he was taken to the hospital as a precaution. The Swedish rookie had minor heart surgery during training camp after he experienced a racing heartbeat.
Maple Leafs general manager Brian Burke said Gustavsson is expected to be fine.
"We're just being ultra-cautious," Burke said.
Colton Orr and Jeff Finger scored in the first period. Joey MacDonald replaced Gustavsson to begin the second and stopped 18 shots to complete Toronto's first shutout of the season.
Orr and Finger scored 3:27 apart. Matt Stajan added his eighth goal in the second. Phil Kessel also had two assists for the Maple Leafs, who lost 3-0 to Buffalo one night earlier.
Carey Price stopped 28 shots in his ninth straight start for Montreal, which has lost three straight.
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.
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 Detroit. … Canadiens RW Matt D'Agostini also returned after missing 13 games because of a concussion and a knee injury.