A win over the league leaders is always welcome, while seeing your star striker deliver the decisive blow to his former club only makes it sweeter - but Manchester United must not get carried away with the win over Arsenal.
David Moyes was sensible when he said he was happy with a crucial three points against the Gunners, but was "not singing and dancing" about cutting the gap between them and the Premier League's top side to five points.
Arsene Wenger was frustrated with his side's first half performance and felt they were a bit unlucky not to get anything from the game after a much-improved second half.
However, neither manager should be getting carried away with the result - most importantly Sir Alex Ferguson's successor.
Robin van Persie sparked debate with his celebration after heading the decisive goal from a Wayne Rooney corner midway through the first half, which has taken much of the publicity away from what was a largely uninspiring encounter - 'Super Sunday' fodder, it was not.
A sense of realism needs to be retained at Old Trafford, as the overall performance of the defending champions was hardly something to get too excited about.
The first half was impressive in how Arsenal players were swiftly closed down in midfield, but that changed significantly when Phil Jones was forced to move back into defence after Nemanja Vidic's concussion.
There were a couple of inevitable opportunities for the home side on the break as Arsenal pushed vainly for the equaliser, but Moyes should be concerned that his side couldn't consistently trouble their opponents in attack.
The Scot must also be mindful that he was up against an Arsenal side at the end of a draining week that took in wins over Liverpool and Borussia Dortmund, so they were unlikely to be at their sharpest even before a number of the squad were struck down with illness.
Per Mertesacker, now one of the north Londoners most important players, was too ill to play, along with the in-form Tomas Rosicky, while Mikel Arteta and Serge Gnabry were also affected but managed to play a part.
Mesut Özil and Nicklas Bendtner have both since had their travels to international duty delayed after getting the flu that looks to be running through the squad.
It was a match that again highlighted the lack of depth in Wenger's squad and he must surely have to at least sign a competent striker come Janaury, but the amount of first team players he has out injured will leave him more optimistic for later in the season.
Concern is warranted, but the Frenchman can be reassured to an extent by the performance of his injury-ravaged side, especially in the second half when they controlled the game - they are also still two points clear in the league.
Manchester United will be encouraged by the win when not playing well, but they got a reminder that further improvement is needed if they are going to hang on to their crown.
It's still early days in the 2013/14 Premier League and both Arsenal and Manchester United will know that they will need to bring more to the table if their title challenges are to threaten to succeed.