Mourinho is familiar with the winger, having signed Salah in 2014 during his time at Chelsea.
"I'm not surprised with Salah's performance this season," Mourinho told BeIN Sports.
"I was aware of his great capabilities when he was with me at Chelsea.
"He improved during his spell at Italy, became more mature and improved physically.
"Players need time to adapt to their new teams. Some of them remain and others prefer to move to another club. It is normal in football."