We don't exactly know what was wrong with him mentally and pointless to speculate either way, but we kind of knew he was out of shape to begin the season and looked completely gassed after 60 minutes every time he played. The issue with those individual training regimes in the middle of the season is that you cannot exactly ease a player in afterwards, all games are important from now on, there are no friendlies or early stage cup games where you can lose and it won't matter much. Reading game may be the one to give him some minutes but considering how awful he was prior to his absence, I definitely would not expect him to jump right into games and start playing significant minutes for a while.
It's also a case we've seen before many times where a player that is off for months suddenly becomes a savior in the eyes of the fanbase. Truth is pre-absence Sancho would not improve this team in any way and would arguably make it worse. Where he's going to be after his time off we do not know but crucial league / cup games will not be the place to test it for now. Once we get more comfortable in top 4, he will start getting more significant minutes off the bench and will probably start a few of the less important games. You don't freeze a player for 3 and a half months and then expect him to turn your season around.