Take VAR out of the equation for a moment.
That call yesterday was a clear indication of the offside rule being unfit for use.
The linesman made his call as he saw it. It is only when we get to freezing an image (which could be stopped at the start, middle or end of contact with the ball, affecting the points of reference.), that we then microscopically examine this image, and come to the conclusion that the defenders hand is discounted and Mata's knee is the furthest point which can legally touch the ball.
How ridiculous to ask a human to make that call in real-time!
Without VAR it is impossible to get that right everytime, yet that is how we are applying the rule.
I don't think VAR in its current state should be used for offside. More importantly, this "legal body part " criteria is adding an extra dimension, to make the job for the linesman impossible. Then he is open to criticism using this photo finish, forensic examination.
Remove the "body part" rule, and let the assistant refs do their jobs, with a chance of being right.