Shaw turned out in United's pre-season clash with Wigan on Saturday, his first appearance since suffering a double fracture of his right leg last September against PSV Eindhoven.
The 21-year-old played what Mourinho described as a "safe 45 minutes" in an effort not to rush his return, and drew praise from his new boss.
"When I found Luke at the training ground, it was a surprise for me because I thought he couldn't be ready to train without any type of limitation, but he did fantastic in the summer. He sacrificed his holiday," Mourinho explained.
"He stayed in Manchester and was at the training ground every day, putting himself in condition. Of course, you lose a bit of sharpness of condition and confidence, that is normal, but he is trying hard."
The former Southampton full-back looked in good condition and not short of fitness on his return, a stark contrast from his first pre-season with the club where he was criticised for appearing to be overweight.