The problem is for him to develop, you need another reliable midfielder in there so that we don't have to depend on Mainoo. That sadly isn't the case, we've invested poorly in midfield and are therefore left with Mainoo being our best number 8 type midfielder.
Obviously, the solution is that we need to buy another midfielder but then again we apparently don't have the current funds to do so. We won't get any money from Casemiro and Eriksen leaving, whatever money we get from Anthony/Rashford leaving would have to be invested in those positions as well otherwise we will be bare there.
PSR is the major issue right now, even if we do sell Antony, his value right now will be so less that the 'profit' we can show from it won't even be that large given the price he was bought at. Selling youth players has been seen as the best way to earn more profit to invest.
So, we want give Mainoo time to develop, which is completely fair, but for that we need to buy another, likely expensive, midfielder but the best avenue we have to generate the funds to buy that midfielder will be to sell Mainoo himself.
It's an absolute clusterf*ck of a situation. Hopefully we can find a situation where we sell Rashford, Garnacho and Antony and can use some of those funds to get a midfielder, without having to sell Mainoo. It would leave us really bare in attacking midfield but we can try to get some cheap loans in for now and depend on Bruno/Amad.