Chile international forward Sanchez showed why United were so desperate to sign him with two assists in Friday's 4-0 rout of Yeovil.
"He was cheap wasn't he? Free transfer! He was free," Mourinho said with a wry smile. "So for that price he's fantastic.
"I think everybody thinks the same in this country. Everyone has to agree that he is a fantastic player and the team that got him has a plus.
"We have a good group of attacking players you could see today we played with three of them, [Jesse] Lingard was on the bench, [Romelu] Lukaku was on the bench, Zlatan [Ibrahimovic] at home, [Anthony] Martial at home. So we have a good group.
"His [Sanchez] quality is a plus and his experience is another plus."
The Manchester United boss also insisted no one was at fault after Sanchez missed a drugs test Monday.
Authorities were apparently not adequately informed of Sanchez's whereabouts, though Arsenal manager Arsene Wenger said the confusion was purely down to the "special" circumstances of completing his move to United.
Mourinho said: "I think Arsenal were very honest in the way they approached the situation.
"We all know we must tell the whereabouts of our players where they are 24 hours per day [clubs have to tell testers if players miss training and the players must give their location for one hour every day]. But I think everyone understands that no-one knew where he was going to be, where he was going to train, where he was going to sleep.
"It was just a period of transfer so I think it is easily understandable and that was no-one's mistake and just a consequence of the moment."