It seems the Ajax man's move to Old Trafford is all but complete.
And now it appears a move has gone through, with 44-year-old De Boer congratulating Blind on his move to the Premier League.
Blind won four Dutch league titles with Ajax and joins United off the back of a successful World Cup campaign working under current boss Louis van Gaal.
Posting from a joint Twitter account between him and his brother Ronald, De Boer said:
Manchester United completed the £59.7 million signing of Angel Di Maria earlier this week, while Marcos Rojo also signed not too long before.
Rojo, however, is still awaiting a work permit - and fans may be disappointed if they want to see Arturo Vidal arrive at Old Trafford anytime soon.
The Chilean recently tweeted that he is '100 per cent Juventus' and is also reported to have told team-mates he is staying at the club.
Van Gaal's side have just two more days after today to complete any remaining transfer business, with the deadline passing on Monday night.
The arrival of Blind, however, may please fans due to the Dutchman's versatility.
United fans - are you excited by the signing of Blind?