The Parkhead boss voiced his concerns over the way agents have influenced the 16-year-old Somalian-born Scotland youth international.
The club helped Feruz's asylum-seeking family to remain in Glasgow but he has turned his back on the Hoops and will reportedly join Chelsea.
"Celtic have looked after Islam and his family very well but it's obvious his head has been turned.
"We've done all we can to keep him here but I haven't seen him since the start of pre-season training.
"We took him and his family out of what you might call a rough housing estate and put them into a nice flat in the West End of Glasgow.
He added: "Islam was going along very nicely with us and had a good agent in John Colquhoun. Then the boy let somebody take over his future.
"His attitude changed dramatically and his attitude towards the club changed dramatically. Yet our club had been so good to him."