Also, Solskjaer didn't cast him out or isolate him in the dressing room (a la Conte with Costa). He just told him Rashford was first choice, doesn't mean he discarded Lukaku. It just meant that Lukaku was rotational, and he feels he should be starting every match at this point in his career (which, props to him, is fair). Also, Lukaku under Mourinho bulked up a lot to try and suit his version of a strong target man. The Lukaku before that, using his physique to bully defenders, dropping deep and running at players (while being deceptively quick) does suit Solskjaer's play to an extent, and if he stays, that's what he will be looking to do.
Letting Lukaku out is not directly related to LT progression, it's more of Rashford suiting Ole's style more. Lukaku is still an excellent striker to have, he just won't be happy if he is not starting almost every match.