"They let him go and we took him," shrugged Wenger.
"I felt Manchester United had made up their mind to let him go before we were interested, to maybe Manchester City or to France, so it was not necessarily us going in there and buying a player."
Wenger revealed Silvestre was unlikely to feature in Saturday's Barclays Premier League game at Fulham as he regains full match fitness from a muscular strain.
However the Arsenal manager hopes the defender will have a big role within the squad.
"He can play centre-back, he can play left-back and that is an area where we were a bit inexperienced," said Wenger.
"He has a great team attitude and he can help us in the dressing room as well with his maturity."