Looking at that clip, it doesn't really matter where Rivers was looking. Rivers had a bead on whoever 89 is. If Ward doesn't throw that block, Rivers gets the tackle.
It always amazes me how anyone except the person throwing the block/tackle knows whether or not it's 'dirty'. To me, 'dirty' would be hitting with intent to do harm, not just stop a play. Some hits may appear to be dirty, but who really knows what was intended, most of the time.