Yeah, Zlatan always guaranteed 30 goals a year, but he also always had a tendency to dissapear versus good opposition, and hasn't played in an actual competitive league in over a decade.
10 years ago he was in Juve when they bought the title, then he went to Inter who had no competition because all the other big teams got hit by Calciopoli. Then he went to peak Pep's Barca, then a short still in Milan. And now 50 goals a year for PSG, in a **** league, playing for a club that has obscene financial advantage. The situation there is absurd, the league may as well be fixed, the whole place makes Bundesliga seem like a very open competition.
He definitely has it in him to bang 15-20 a year in the Prem though, anything above that is a bonus. In any case, on a free transfer with 1 year contract, it's a no-risk transfer. He'll move a lot of shirts, score some goals, if it works, great, if it doesn't, the marketing gains for United will probably cover his wages.