Late delivering carrier is responsible for fixing onward transportation... If UA is operating the delayed flight they'll ultimately be responsible for fixing it, but res likely can't/won't by policy until the misconnect is certain (e.g. until you're in SFO or you're not in SFO and the other flight has departed).
At this point your best bet is to talk to an agent at the originating station (who has a better situational view and can be more positive they you will misconnect) or call the issuer of the ticket