Sanchez last week completed the drawn-out switch, with Henrikh Mkhitaryan moving in the opposite direction -- a deal which left Beckham pleasantly surprised.

The former England captain, speaking in Miami at the launch of his new MLS club, told Press Association Sport: "I still can't believe it's happened, to be honest.

"When he's been part of a club as big as Arsenal and then he moves a few hours away up north to the biggest club, it's astonishing to see.

"We all wish him luck. I'm a Manchester United fan so I am excited about having someone of his talent wearing a Manchester United shirt -- so it's exciting times for us."

The 42-year-old Beckham also took the time to back Jose Mourinho as the right manager for his old club. The Portuguese last week signed a new contract that runs until 2020, with the option for a further year.

"Of course he's the right fit for Manchester United," Beckham said. "We're talking about Jose Mourinho. He's one of the best managers in the game.

"Manchester United has had success over the last 25 years because we have had stability that has run throughout our club.

"Sir Alex Ferguson -- the amount of time he was manager of the team -- that's why we were successful because we had stability that ran through the club and that's what we need going forward.

"Everyone is going to have their own opinion of [Mourinho] as a manager, him as a person, him as an individual. But is he the right person to lead Manchester United? Yes, of course he is."