Is it legal..... knee a fighter to the head when they have a knee on the ground?

Jardine had a knee on the ground when he got hit.

didnt hit the head i dont think, more neck/shoulder.

looked like he got knee'd in the chest

the fight should of been stopped before the knee anyway

Jardine was done after first punch he took in clinch after he dropped Alexander.

I understand about what would have happened, but the question is "is it legal?"