So two different things: you use your US passport to board the plane to Canada. No issues there. YOu're not required to identify yourself as a PR to the airline.
You MUST* enter Canada - that is, at the port of entry (YYZ passport control) as a PR, i.e. you show your COPR or whatever other docs (and your US passport as just personal identification). Even if you don't have any of that stuff, do it verbally. (Of course modern age you're going to have something, even digital copy). UCI# (which is also on the PR card/COPR) helps them identify you as a PR.
Now, what will they do about you not having a PR card? Almost certainly nothing esp if the reason is obvious (like havne't received it yet). The worst they can do is take you to secondary and slow you down a bit. And the reality is, apart from the minor hassle of looking you up, there's no issue at all.
*There might be some argument about whether this is required by law. I'm not interested in that debate. For almost all PRs esp recent - let's just say they're going to figure you out that you are a PR, and it's a bad idea to look like you're hiding it. That's my backing arg for 'must' in this context.