Apparently they will as offside is to be treat differently I think as it's not down to someone's interpretation of what happened. This was said after Mata's disallowed goal for Man Utd against Huddersfield when his kneecap was just offside.
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Offside is an objective decision
This is a crucial point, and why those shouting that the incident was not "a clear and obvious error" are arguing over an irrelevance.
The "clear and obvious error" consideration within VAR will only be used for subjective decisions -- penalties, fouls, possible red cards. They are decisions which are open to interpretation, and they are also the calls that the VAR can ask the match referee to look at again on his pitchside monitor.
But offside is different. You are offside or you are not. It's a factual decision based on the position of, usually, two players on the pitch. The same goes for the ball going out of play, it is objective and will never be judged on being "a clear and obvious error."
Even though the VAR line was at the feet, they've still said the Southampton's player's shoulder (part of body a goal can be scored) wasn't enough to play the Derby player onside. If you draw extra lines on to get a better idea you can see it's still offside as the blue line is down from Southampton's player's shoulder to his foot to get an idea of where his shoulder is in comparison to the red line of the Derby player. It's a bit daft really overturning such close decisions but offside is offside and it's impossible for the on pitch officials to see stuff like this.
It's daft how it takes so long but I think in time they will get faster as it's still in its early days.