Silva, 31, joined the Spanish giants as a free agent following the expiration of his Man City contract earlier in the summer.
The midfielder has since been away with Portugal for the 2026 World Cup but starting just one of their five games.
He came on at half time during Real Madrid’s 2-1 pre-season win over Ferencvaros and looked like he needed to get up to speed with his new teammates.
Speaking to Real Madrid TV after the game, Mourinho admitted that Silva isn’t quite at the needed physical level.
“He is very important, but the poor guy is one of those players who benefits mentally from not doing anything during vacation, and he arrived for preseason at a fairly low physical level,” Mourinho said.
“'He has to improve. But he is an incredible player.”