Diouf had levelled for Leeds in the second half, but he was red carded shortly afterwards for making a gesture to Brighton fans and Leonardo Ulloa hit a late winner for the visitors.
Will Buckley had earlier opened the scoring for Brighton before Rodolph Austin and Inigo Calderon were sent-off in a bad-tempered clash at Elland Road.
When asked about Diouf, McDermott said: "I can't understand him and maybe he can't understand himself.
"What he did is not clever. He has got to learn because he is going to miss three games now. He is really upset but he has to be accountable for his actions.
"These things are dotted around his career. He can be a match-winner and I've told him he has to let his football do the talking.
"If you run 50 yards and make a gesture, that is not a heat of the moment thing. I believe 100 per cent he can learn from this - he has to.
"Austin just bumped into their player so no way was it a sending-off offence and we will appeal against it."