"We'd been monitoring the situation when he turned down Southampton, but we thought it was going to be pretty pricey. However, his contract was up next year so we moved in on the Thursday or Friday. We had to do it quickly."
Büttner is capable of providing a goal threat from left-back after initially playing further up the field.
"Alex has made good progress in the last year and is on the verge of the Dutch national team," added Sir Alex. "He's a good last third of the field player who makes chances and can score goals. He's scored five or six goals last season which, for a full-back, is quite good. We're delighted to have him here."