Think they are correct. At this point he's just short of world class. Costa and Ibrahimovic have everything, intelligence, heading ability, goalscorers and both are superb at holding the ball up.
Lukaku has the size but at the moment is nowhere near them levels of protecting the ball, making it stick and holding onto it until you have support. He will do it now and again but he's very hit and miss at the minute. 90% of teams will sit back against Chelsea and Man U and Rom may just struggle a bit in them situations.