"Lack has played really well the last two games and deserves the start," Canucks coach John Tortorella told the team's website.
It will be Lack's third straight start. He had a 20-save shutout Wednesday against the St. Louis Blues and then made 22 saves on 23 shots in regulation and overtime, plus six saves on seven shots in the shootout in a 2-1 loss to the Minnesota Wild on Friday.
In 24 games, Lack is 9-8-4 with a 2.05 goals-against average, .925 save percentage and three shutouts.
The decision to start Lack leaves Roberto Luongo as the backup. Luongo lost five straight games before the Olympic break and played just once for Canada during the 2014 Sochi Olympics as Carey Price's backup. His last NHL action was Feb. 8 against the Toronto Maple Leafs.
For the season Luongo is 19-16-6 with a 2.38 GAA, .917 save percentage and three shutouts in 42 games.
The Canucks also will dress seven defensemen again, with Raphael Diaz drawing in and forward Jordan Schroeder a healthy scratch.
Vancouver forward Brad Richardson, who missed practice Saturday for personal reasons, is expected to be in the lineup.
Here are the expected lineups for the Canucks and Senators:
Injured: Mark Stone (arm)
Tom Sestito - Zac Dalpe