Lukaku is still poor at things, some of which will never change. Doesn't stop him being boss. You can't be poor and score loads of goals, It just makes said player a goal scorer.
Felli obviously matured as a player and got better, but was in no means shit when we signed him, he was just different. Mainly because we played him up where he couldn't effect the game as much.
I do think fellaini is a fairly poor comparison - I actually do remember him being pretty bad in the first few appearances but he did always have that tactical reason to get included in a team - his size and ability to bring balls down from the air or flick it on, defend and attack on set pieces etc.
I really struggle to see what tactical advantage (size, pace, engine, aerial ability) that Sandro offers at this moment in time which makes a very strong case for him being included in the starting XI to counter his shortcomings so it seems so unlikely that he will get time in the 1st team to play his way into some form. he just doesn't seem strong enough at any one thing to make a case for playing him and dealing with the time required to get him up to speed