Neither of them would be losing to European canon fodder for starters, and would be progressing out of the group stages easily and not allowing their respective teams to fall into the Europa league. And unlike Ole they've racked up an enviable trophy count.
But it's not just that. The whole 'but Sir Alex also had X, Y, Z games/tournaments/signings is a debate killer not because it's convincing but because in 26 years of managing United, of course he had pretty much every sort of result imaginable. If the standard is 'be worse than Sir Alex has ever been' then none of our managers should have been criticised, ever.
Mourinho is sixth on the table in December, long way behind the leaders or even the CL places? Sir Alex finished 11th in 1989 and 13th in 1990!
Van Gaal failed to get out of the group stages of the CL? So did Sir Alex in 2005 and 2011!
Van Gaal put us to sleep with his football, scoring only 62 goals in 2014/15? Well Sir Alex's United only scored 58 in 2004/05.
Moyes got utterly annihilated at Old Trafford by both Manchester City and Liverpool? So did Sir Alex in 2011 and 2009, respectively.
It's a pointless argument. Just because a manager is very much capable of replicating the lowest points of Sir Alex's tenure, it doesn't mean that he's capable of matching the highest ones. It's not a meaningful defence of anything. Sure, you can claim that defeats like yesterday's are not the end of the world and use previous similar debacles to underline that point that the club had not collapsed back then either - but nevertheless, those similar defeats were still bad results. They were still low points. They still warranted criticism.
Of course, if we do indeed fail to get out of the group stage of the CL again then Solskjaer will have done something that Sir Alex never managed: getting eliminated in the group stage twice in a row. I hope it doesn't come to that.