With David and Buchanan being 2 of their best players, I'm pretty sure the Canada plan was to rest them for a much more winnable game vs El Salvador. I think Canada would tell you they were pretty shocked to be 1-1 in midway through the 2nd Half without those 2 so they thought, what the hell, lets put these guys in and maybe win this game.
I think their plan was always to put those guys out there at some point unless the game got out of hand and they didn't think they could make a difference. Those guys were warming up to potentially come in before Canada scored, and I think that even down one with 20 or 25 minutes left they would have thrown them out there and tried to get an equalizer.