All we can go on are rumors, but it was rumored at the time that Ralf wanted to get rid of Lingard and bring in a decent CDM in January, but the oracles who run United said no to both. We can't be sure of the truth of these rumors, but if they are true surely Ralf gets some points for trying to strengthen the squad when the chance to do so was before him.
Ridding Lingard and bringing in a decent CDM may not have prevented the downward spiral of the club, no doubt, but it might have.
But from a broader perspective, it's hard to see what more any interim manager could have accomplished given what he was given after the string of capitulations that led to Ole's sacking. First, the steaming pile of shit. Then then Pogba drama. Then the Lingard drama. Then the Greenwood drama. Then the abysmal form of both Rashford and Maguire, which was well baked in before Ralf accepted the scepter. Then club management saying no to any new bodies in January (which might have been prudent, to be fair).
We could have gone in for Conte, which I supported at the time, but it remains an open question as to whether that would have been the right thing to do. As it is, we landed the EL spot we were destined for by the time we brought in Ralf, and instead of being stuck with Conte (he's got plenty of doubters) we'll soon have a manager whom many believe (I'm one of them) has the tools to bring us back to where we belong.