He was right to disallow the home side's 'goal' after 27 minutes because Javier Hernandez's head was closer to the goal-line than the head of the second last defender.
Right on both counts: Hernandez misses out (above) but did get his goal (above)
When the lawmakers defined which part of the body should be used to judge offside we all thought they were being pedantic - after last night it is clear why the regulation is important.
The same assistant referee got the law right again by giving the benefit of the doubt to Hernandez when even slow motion failed to show clearly if he was offside when scoring United's opening goal from Ryan Giggs' pass.
STAMP IT OUTNani twice displayed poor sportsmanship last night, waving imaginary cards at referee Olegario Benquerenca in an attempt to get Michael Essien, Alex and John Terry booked.
Chelsea also pressurised the official, crowding him when Terry was shown a yellow card for catching Javier Hernandez with a stray arm.
More from Graham Poll. Graham Poll: Refs want to show red card to crazy rules. but they can't11/04/11 Graham Poll: How ref Lee Mason went from wonderful to woeful at Blackpool 10/04/11 Graham Poll: Goal-line official should have helped referee with penalty row07/04/11 Graham Poll: About time Roo! FA finally show some backbone04/04/11 FA must take the hard line with Rooney and ban him for three games. But they won't.03/04/11 Graham Poll: Show some Respect. this abuse of referees must end now31/03/11 Graham Poll: Perhaps Rooney was already in holiday mode in Cardiff.28/03/11 Graham Poll: Call that a ban? Sir Alex Ferguson still able to have his say21/03/11 VIEW FULL ARCHIVE Manchester United 2 Chelsea 1 (agg 3-1): Hernandez and Park heap more heartache on Blues CHAMPIONS LEAGUE LIVE: Manchester United 2 Chelsea 1 - the action from Old Trafford as it happenedMartin Samuel: Thanks for nothing, Fernando. Now take rest of the season offAll the latest Manchester United news, features and opinion
Explore more:People: John Terry, Michael Essien, Ryan Giggs