I had a very similar situation earlier this month... UA flights booked YYZ-IAD-LAS. The inbound flight into YYZ went MX and was delayed for 4 1/2 hours, which of course would have caused me to miss my connection. I called the UA phone agent and eventually (after he got the AC people on another line), they confirmed me onto the AC YYZ-LAS non-stop flight the next morning along with the hotel in Toronto and meal vouchers, since it was impossible to get any other flights in to LAS the same evening (and that included other AC routings via DEN or SFO).
So yes, when it is something that is under THEIR control (such as MX), they will most certainly do what they can to get you to your destination, but you should also definitely have in mind the specific flights to recommend to them.