Bostock left Crystal Palace, while Warnock was in charge, for Spurs.
Warnock wrote in The Independent: "He's been on loan to Brentford and Hull without establishing himself at either and is now back at Spurs without a squad number. He'll be 20 in January, but in four years he has started just 18 league games and he hasn't played a senior match for 11 months.
"I feel sorry for the lad, but if he had stayed at Palace he could have played close to 100 Championship matches by now which would have set him up for the rest of his career. As it is, no one seems to have done well. Tottenham probably feel they haven't got value for money, Palace lost a player on the cheap – and it doesn't look as if they are going to get any of the add-ons – and the boy, who's the most important person in all this, isn't playing football."