Smalling scored what proved to be the winning goal in a 2-1 triumph at Watford, ending the host's 100 per cent start to the season, even if late pressure meant it was a close shave for United.
Aside from the goal, the defender's display was impressive, as he dealt with Troy Deeney particularly effectively at Vicarage Road, proving to be a cut above the opposition.
But, after sporting a new 'do' which appeared to be a combination of cornrows and bantu knots, Smalling should not be expecting any style tips from Mourinho.
"He was good, the only thing I don't like from Chris at this moment is his haircut, but I am nobody to tell him that," Mourinho joked.
"But he's playing really well, also Victor [Lindelof], and I repeat, Marouane [Fellaini] is really important for them in two difficult matches, matches of two-against-two, because Burnley and Watford play two strikers, two powerful strikers.
"They can hold the ball, they can make runs, they are good in the air, they are physical, clever, so I think these three guys [Smalling, Lindelof and Fellaini] were very important for us today again, and when a central defender can score a goal, even better."
Fellaini made his second start of the season after impressing against Burnley last time out, and Mourinho is delighted with the Belgian's form, while also crediting his "simple" style of play.
"We made defensive mistakes against Brighton and against Tottenham," Mourinho said. "Against Brighton we didn't deserve to win that match, but against Tottenham I think we did deserve to win it.
"But six goals and defensive mistakes, and then we go to two matches away — difficult matches, different stadiums, Burnley, Watford — and I felt that my central defenders they needed, especially in this moment of a bit of instability, they needed somebody to support and to give them the first wall; physicality.
"But Marouane is giving us more than that. He is giving us that, but he's giving us also quality and simplicity in his football. He's playing simple.
"He's playing simple and well. So I'm really happy. I'm really happy with Marouane."