The Belgium international has moved to Deepdale on-loan.
Lindsay told the Lancashire Evening Post: "We've agreed a deal with United for Ritchie de Laet - it's been sanctioned by them. We've just got to finalise the length of the loan then everything will be done.
"He's joining us for training today and will be available for the QPR game.
"Ritchie is a versatile defender, he spent a month on loan with Sheffield United recently so he knows the Championship."