United boss Van Gaal added Matteo Darmian to his ranks in the summer, with the Italian right-back joining from Torino for around £12m, as reported by the Express, but the Dutchman failed to sign a top-class central defender in the transfer window.
Daley Blind is currently operating alongside Chris Smalling at centre-back for the Red Devils, but after Tuesday night's 2-1 reverse to PSV Eindhoven in the Champions League, Keane remains unconvinced that the pairing is a long-term solution.
Luke Shaw's horrendous leg break could also see the versatile Blind move to left-back, and Keane wants Van Gaal to dip back into the transfer window when it re-opens in the New Year.
Speaking to ITV Sport, the former United captain gave a typically forthright assessment of his old side's performance against the Dutch champions.
Keane fumed: "Defensively I still think United are all over the place, The money United have invested, going forward they will be fine and in midfield they are not too bad but I think they are crying out for centre-halves, definitely one, possibly two.
"As long as Blind, and even Smalling, I'm not sure about him, or either Jones are playing centre-half, particularly in the Champions League against bigger teams [they will struggle].
"It was a poor result, poor performance and giving away bad goals. They had chances to get something out the game. Disappointing night for United."
United return to action when they travel to Southampton in the Premier League on Sunday afternoon.
The Red Devils have collected 10 points from their five top-flight fixtures so far, with that return enough to have them sat in third place - five points adrift of table-topping Manchester City.