Have just watched the highlights for the first time and to me its a pre-meditated clear-cut push rather than the usual 'six of one, half a dozen of the other' tussles that go on in the box week in week out. Therefore, my verdict is that its an obvious foul. What crystalises this in my mind is that if Hoedt had pushed Lukaku in the same clinical fashion, the ref is likely to have given a penalty.
(I'm aware that my opinion may be influenced by Saints supporter bias, but I've tried to be as objective as possible in my assessment here)