He is the second Dutchman to join Old Trafford this week, following the £25 million swoop for striker, Robin Van Persie.
The 23 year old has been signed on an undisclosed fee from Dutch side, Vitesse Arnhem. He made over 115 appearances for the club in the last four years, and attracted interest from various European clubs this Summer.
Ferguson said: “Alexander is one of the best young left backs in Europe and we’re delighted to sign him. He’s someone we’ve been watching for a while now. He gives us some really exciting options in that position.”