Robin van Persie's match-winning goal against Arsenal was a bitter pill to swallow for the travelling Gunners faithful.
And, after he celebrated in some style at Old Trafford yesterday, it left many fans angry at his supposed lack of respect to his former club.
However, despite the hatred for their former star, fans of the north London outfit should realise that Van Persie's celebration showed respect to his ex-employees.
Last season, when the Red Devils cruised to the Premier League title, the Dutchman struck home and away against the Gunners and held back his celebrations. Instead he seemed more apologetic as he helped Sir Alex Ferguson's team take four points from Arsenal.
And perhaps last season he failed to celebrate his goals because he almost felt sorry for his former employees. In last year's campaign, the striker knew his former side weren't a threat to his Manchester United team.
But this season is different. While United have seemingly failed to improve, Arsenal have developed into a new serious threat.
And the importance of yesterday's game cannot be underestimated. If the Gunners had beaten David Moyes' side, then Arsenal would have been 11 points ahead of the champions and United's confidence would have hit an all-time low.
But, instead, the Red Devils picked up a crucial three points and now sit just five points off the pace at the top.
That's why Van Persie had to celebrate. He now knows his former side are a threat to his and Manchester United's Premier League title.
He proved that his former club had got under his skin slightly. He was more than happy to take the abuse and criticism last season when Arsenal were failing to cement any sort of challenge to him.
But now, his former side are a rival to him and his team, so he had to react and had to show no mercy.
Of course he will be criticised and many will suggest he shouldn't celebrate against the team that helped make him.
But, yesterday as he struck once more against his former club, he really proved Arsenal are behind him but also a major threat to his success at Old Trafford.
When the two sides meet again in February - this time at the Emirates - Van Persie will once again be ridiculed from the Gunners faithful, but this time he will return a serious rival and will once again look to break Arsenal hearts.