To be fair, I read that the three were not unanimous in their decision (not sure where I saw that, one of the online papers I think). Which means it was a 2-1 split, but don't know which way. To be even fairer, the test they have to apply was 'would they have given a red card / was it a red...
Looking at the various clips, the ref can't really include it in his match report as being seen by him. Watch him, he's clearly followed the flight of the ball away from the six-yard box, and so would not have been looking in Lukaku's direction. Whether the linesman saw it is another matter.