Gakpo, 23, was strongly linked with an Old Trafford transfer and came close to a deal last summer.
But the Dutch international was unable to resist when Jurgen Klopp came calling after the World Cup.
Gakpo, who scored 34 goals in his final 18 months in Eindhoven, has looked uncomfortable playing in a variety of roles for Liverpool.
And PSV boss Van Nistelrooy has reportedly now hit out at Gakpo for ignoring his advice about waiting until the summer for a “dream” switch to United.
The former Red Devil said: “Gakpo’s dream move was Man United. I advised him to wait until the summer.
“Something happened and he refused my advice and listened to Virgil van Dijk who tempted him to join his team.
New Holland boss Ronald Koeman also feels Gakpo has made a mistake joining Liverpool.
He added: “Gakpo has ended up in a team that is not doing well. If you don’t score or you’re not important and you don’t win matches, it’s very difficult, especially for a young player.
“If that someone aged 28 with experience it would be different. The level in England is higher than in the Netherlands.
“But they are also young boys, aren’t they? Like Ryan Gravenberch going to Bayern Munich and not playing. Then that is difficult.”