Van Gaal, speaking with BILD, rapped Mourinho for the way he treated Bastian Schweinsteiger after he took over United.
Van Gaal said: "Schweinsteiger was of course older, but not too old. His body could not keep up with the high standards in the Premier League.
"Bayern sold him to us as a fit player, in fact he was so physically.
"How Mourinho treated me after him, Schweinini still did not deserve that. But it explains the situation, how it all stood around Schweinsteiger. That's a shame because he's a player like Luis Enrique, Mark van Bommel or Philipp Lahm. These are guys who are always present on the pitch."