Ling watched his Orient team lose 2-0 in London on Saturday, realistically ending their chances of finishing the season inside League One's top six, and he admitted United's dominant performance had marked them out as serious promotion material.
He said in the YEP: "I think Leeds will be in the play-offs now without doubt. It's a bit of a lottery but Leeds have got enough to do it. But then Nottingham Forest's supporters would probably say they've got enough as well. It'll be very interesting because they'll definitely be two of the four teams in there."
Ling conceded that his team had been beaten by a more proficient side at Brisbane Road, saying: "It was men against boys really. They moved the ball better than us and had more purpose to their play. Let's be honest, the best team won and they showed the gulf in class."