United boss Louis van Gaal has been linked with a move for the Ajax defender, who impressed for the Netherlands at the World Cup.
The Red Devils have lost Nemanja Vidic, Patrice Evra and Rio Ferdinand this summer, and Van Gaal has so far used academy graduates Tyler Blackett and Michael Keane alongside Phil Jones and Chris Smalling in defence.
United's conversion to a three-man backline for this season has made the need for defenders even greater, and the Red Devils signed Marcos Rojo from Sporting Lisbon for £16 million last week.
Rojo, who can operate as a central defender and a left-back, has been added alongside Luke Shaw, who was signed from Southampton for around £30m, and Carragher finds another pursuit of a left-sided player puzzling.
Speaking on Sky Sports' Monday Night Football, the former Reds star said: “I think it’s strange to buy Rojo and Luke Shaw and then you’re looking for another left-sided defender [in Blind].
"You need players in your squad, I get that but it does seem strange to buy that many players in that position. Rojo is a left-back who can play centre-back, so another one [is strange]."
United have so far spent around £76m on the deals to land Shaw, Rojo and Ander Herrera, but that fee is set to rocket with confirmation of a British-record transfer to sign Angel di Maria from Real Madrid imminent.
The Argentina international winger will make the switch for a widely-reported £59.7 million, while former Manchester City midfielder Nigel de Jong has also been linked with the Old Trafford club.
United are yet to win under Van Gaal, after an opening-day defeat to Swansea City was followed up by Sunday's 1-1 draw with Sunderland at the Stadium of Light.