Manchester United boss David Moyes felt his side performed to their potential in opening their Premier League title defence with a 4-1 win at Swansea.Robin van Persie and Danny Welbeck scored two goals apiece to mark Moyes's first league match in charge of the champions with a ruthless victory."It very much was a Manchester United display," he said. "We were clinical in attack and I am very pleased."This is a tough place to come and we won well in the end."Moyes left Everton to succeed Sir Alex Ferguson in the summer following his fellow-Scot's retirement after 26 years at the Old Trafford helm.United's pre-season results were mixed but Moyes collected a first piece of silverware as his team won the Community Shield last Sunday, while this result puts them top of the table on goal difference.Van Persie opened the scoring with an athletic strike before Welbeck tapped in to double the advantage two minutes later.It was 3-0 when Van Persie unleashed a fierce strike and, although Wilfried Bony slotted home to pull a goal back for the League Cup holders, Welbeck sealed the point with a superb chipped finish."Yeah it is [a relief] because you always want to win your games and as manager of Manchester United the job is to win," said Moyes."Swansea had a lot of the ball, passed it well and are well managed under Michael Laudrup but we limited them to few chances."We could have played better but we're happy to win 4-1."Moyes brought on Wayne Rooney after 62 minutes and the England striker, who is understood to be intent on leaving United, played a role in his side's third and fourth goals."He got 65 minutes in the [England] game against Scotland and I felt he could do a job today if he came in," Moyes continued."He's not ready yet physically as he's not done a lot of training but he came on and had a great run. It will be great for Wayne to get those minutes on the pitch."People know good players and Wayne is a good player and people want them in your team."