Arsenal's problems were brought into brutally stark relief at Old Trafford as they suffered a disembowelling at the hands of Manchester United, with Wayne Rooney scoring a hat-trick in an 8-2 victory for Sir Alex Ferguson's side.The result was the most desperate defeat Arsenal have suffered in the Premier League, with their previous worst a 6-1 loss at the same ground in February 2001. On that occasion, as on Sunday, the root cause of the collapse was a hopeless, makeshift defence, but the current malaise enveloping Arsenal threatens to be something altogether more serious.Rooney ran riot with two direct free-kicks and a penalty, bringing up his 150th United goal in the process, Ashley Young scored two brilliant curling efforts, Danny Welbeck, Nani and Park Ji-Sung added further goals and Arsenal's futile replies came from Theo Walcott and Robin van Persie.The Arsenal captain had missed a penalty in the first half but United comprehensively dominated throughout and could have had more, with Rooney unlucky to hit the post in a quite wonderful performance from the England striker.Carl Jenkinson was also dismissed - the third red card shown to Arsenal in three games - and the result puts United top on goal difference, leaving Arsenal in 17th with one point from three games after conceding eight goals in a league game for the first time since 1896.Following confirmation that Bacary Sagna and Thomas Vermaelen would both be missing from a team already ravaged by injury and suspension, and that Francis Coquelin would be asked to anchor the midfield on his league debut, a certain 6-1 defeat sprang to mind.As it transpired, such a scoreline would have been favourable. Arsenal appeared ill at ease from the start and were given a warning after six minutes when Welbeck sprinted away from Laurent Koscielny before firing back across goal and wide.With United superior all over the pitch, Arsenal were somewhat fortunate to see Andrei Arshavin escape with only a yellow card for a late and high challenge on Phil Jones. When the Russian then clattered into a rash tackle on Young, it was a mystery how he remained on the pitch.Arsenal's strained defence gave way after 22 minutes: Anderson lofted a pass over the top of the defence, and both Koscielny and Johan Djourou completely failed to deal with Welbeck. Either man could have headed clear but instead contrived to allow the forward to head home from close range, enraging Wojciech Szczesny.