Man Utd boss Ferguson pleased with "terrific addition" Buttner
The Dutchman was a surprise capture from Vitesse Arnhem.
Ferguson said: "We've been monitoring Alex for a while. He's a straight-forward attacking, quick, full-back. He will be a terrific addition to us.
"We have to protect the future. That is sensible. Evra is 31 and Alex comes now to challenge that position."