I've thought both scenarios out in my head. Obviously with him going he's still more than likely the second QB taken and the best case scenario is that he goes to a team that's not really in dire need of a QB but needs one for the future (similar to the Ravens picking Lamar) so he gets to sit behind a QB for a year or two until his number is called upon. All of this is barring any set backs.
The other scenario of him coming back is risky, let alone injury risk but risk of slipping further because let's say he comes back next season and plays (don't know if he'll be ready by USC but let's assume here), if he looks really rusty and has just an average season, he definitely slips behind Fields and Lawrence in 2021 and could slip out the first round, on the flip side if everything goes well and the rust is not there, he could possibly trump Lawrence and be that #1 pick when everyone assumed for three years it would be Trevor.