The Portuguese tactician took over as manager of United last summer and with that he returned to management months after been sacked from his position at Stamford Bridge in December 2015. He was shown the exit door at Stamford Bridge, six months after he guided them to Premier League triumph in May 2015.
Under Mourinho, the 20-time English champions won the EFL Cup in February and are in contention to win the Europa League. They have not enjoyed a good run in the league as their failure to convert draws into victories has hurt them this season.
United are fifth on the table with 60 points after 31 games and trail fourth-placed Manchester City by four points, but have a game in hand. Nevin has backed United to qualify for next season's Champions League after heaping praise on Mourinho for his tactics in his side's victory over his former club.
"I have to say I was surprised by how shocked people seemed to be by the result on Sunday at Old Trafford," Nevin told Chelsea's official website.
"It is worth getting it straight, Man United are on a monumentally good run without defeat. For all that they draw too many games, they have the biggest squad in terms of quality in the league and they were playing at home.
"They had everything to play for in that they are striving for that Champions League place, which I have always felt they will get, and have a manager who is quite good technically and tactically," he added.
Speaking about Mourinho's stay at Stamford Bridge, Nevin said, "Actually he is better than quite good at the tactical side as we know more than anyone, but his inside knowledge of the Chelsea team was the killer on the day. He knew where to strangle our creativity and he managed to fire up his own players.
"On top of that they got lucky with both goals for different reasons (if the referee has missed Herrera's hand ball in the build-up to the opening goal, the officials helping him surely must have seen it) and our lads had something of an off day as a group, not helped by Thibaut Courtois and Marcos Alonso being unavailable."