In a 3-0 win for Usuthu over the champions last night, Morton opened the scoring with a stunning strike from range at King Zwelithini Stadium.
However, the on-loan midfielder was sent off late into the game by referee Eketsang Setloboko for shoving Mhango to the ground after being spat at by the Malawian.
"I thought it was very unfair but the initial instigator, which was Mhango, I think he spat at him," Johnson said.
"If someone spat at you in the face and you've been running for 75 minutes your head is not all there. You are fighting for the game, you want to control the game and Mhango does that. I think it is very unprofessional of Mhango to do that.
"But at the same time maybe Michael Morton's head just slipped a little a bit, and of course the ref gave him the red card for retaliation.
"I think Mhango is not a good professional and he's not a good advert for South African football."
Unsavoury incidents aside, Johnson was pleased with how his charges responded from a 4-1 thumping by Durban rivals Golden Arrows on the weekend.
"If you treat athletes like athletes and you show them how we got beaten by Golden Arrows, then they'll understand this is not a do-or-die situation," Johnson said.
"If you guys were at the game... 4-1 yes and they also beat Wits 3-1, but 4-1 for me, I think we had better moments for me to work on.
"But like I said, if you treat athletes like athletes and not like dead rats, then they'll give you the performance of tonight [Wednesday] and they'll give you more in the future."
The Premiership returnees next host Johnson's struggling former employers Platinum Stars on Saturday.