I don't understand selling Lukaku and then looking for a replacement to play second fiddle to Rashford, it seems like a completely unnecessary and risky sideways move at best.
I could understand it if there was an available and achievable target who would be seen as an improvement on Rashford but buying players who are not deemed better than your current starter (it's fine if your current starter is an Aguero or Kane) is the quickest path to ending up with a very mediocre squad very quickly (look at Wenger's latter years at Arsenal as the best example and players like Fred at United, Drinkwater at Chelsea)
Say Ben Yedder who has been linked. He will want a 5 year deal on something like £120,000 a week and Rashford will be the starter. All that means is if and when an elite forward becomes available United would be stuck with trying to offload Ben Yedder who won't be willing to take a cut in salary and that limits the options to move him on. Like Drinkwater, Chelsea actively tried to sell him last summer but nobody would match his wages and he was happy to sit out a year basically rather than take a paycut (fully entitled to do so)
So basically, a club looking to improve should only ever sign three categories of players :
1. Young players with plenty of room for development (in ability and value)
2. Established players who are going to be a backup to your elite starter
3. A player that is better than your current starter
I've not seen a single touted Lukaku replacement that fits any of those categories so in my eyes it makes zero sense to sell (even more so considering in my opinion any belief that Solskjaer has a playing identity or style is fanciful at best so to sell a reliable goal scorer, more reliable than Rashford, because he doesn't fit said mythical identity or style makes even less sense)