The 23-year-old, who is expected to undergo a medical this afternoon, was included in the provisional Netherlands squad for Euro 2012.
He told reporters on Monday: "If a week ago, you told me that together with Robin van Persie I would be playing at Manchester United, I would have driven you straight to the madhouse.
"One month ago Manchester United sent an email expressing their interest, but in the last four days it became concrete. I have not spoken yet with Sir Alex Ferguson, but if all goes well that will happen."
Buttner is expected to provide cover for Patrice Evra at left-back.