The former Vitesse Arnhem left-back completed his move to United yesterday.
Streuer admits they considered a move for Buttner earlier this summer.
"He's the greatest talent in eastern Netherlands. The outside world seems to think the transfer to Manchester United is perhaps strange, but for his mentality, Alexander can make it there.
"In his position, he's one of the best in Europe. Left-backs like Buttner are very hard to come by. He can defend well and is also attack-minded in possession."