It is understood the striker has arrived at Old Trafford complete a year-long switch after the club beat rivals Manchester City and Arsenal to clinch a loan deel for a fee of €12.5 million and will pay most of his €380,000-a-week wages.
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"Best signing of the window? Obviously, Falcao sticks out and I am a fan of the striker but I didn't think it was the position United needed to strengthen," Owen told Sportslobster.
Falcao's move follows the signings of Ander Herrera, Luke Shaw, Angel Di Maria and the soon-to-be-announced Daley Blind.
Owen has questioned the balance of Louis van Gaal's squad - with the manager yet to record a win in a competitive game at his new club - and argues that the defensive cover is still a priority position to strengthen.
"I think United needed to strengthen defensively but the players just aren't available at the moment," he continued.
"People say Ron Vlaar at Aston Villa but no-one rated him until the World Cup and so I am not convinced.
"I think the three in defence isn't working at the minute and so, if I was Van Gaal, I would go back to four in defence.
"United now have so many attacking players that I am unsure how they are going to fit them all in. I think this is why they originally started with the 3-5-2 formation, so they could accommodate loads of attackers, but now I think Van Gaal may sacrifice [Juan] Mata."