Yes. This is called Original Routing Credit (ORC). UA will give you the miles as if you’ve flown the UA flight. Simply call the MileagePlus number and ask for it - specify the term ‘Original Routing Credit’ as using something else may confuse the agent - and explain that your flight got canceled and UA rebooked you. Your ticket should have been marked ‘invol’ - meaning you got ‘involuntarily re-routed’ and it wasn’t your fault.

In this narrow type of case, you should also be able to claim mileage for the flight you did take on AF also - so you can essentially double dip in credit. You can get this on AF, DL or any sky team or AF partner by retiractively crediting based on their policy. IME, if you credit to DL, you can use their online request to do it - you’ll need to enter the updated ticket number. While I’ve never run into issues doing this, I’d wait for the UA miles to post so you can ensue those credit for status. Hey typically don’t (or can’t) check on the existing credit - but if for some reason that changes, you’ll get it where it’s most important.

My last experience doing this was in December, but not even on a UA flight. I misconnected at BKK on TG for a BKK-TPE flight, and TG re-booked us on CI. While I certainly won’t recommend a CI experience - I called UA and thought it was a logshot for ORC since the TG flights weren’t booked nor flown on UA - but they were able to credit. I then went to Skymiles and credited the CI flight to DL.