Chris Coleman admitted that Didier Ndong 'gave the referee a decision to make' following his sending off.
Ndong was shown a straight red by Andrew Madley for a challenge on Junior Hoilett. Sunderland were 1-0 down at the time and conceded a second within minutes.
Coleman felt the Gabon midfielder was perhaps unlucky but ultimately had no qualms with the decision.
He said: "The sending off, sometimes they’re given, sometimes they’re not. I think the referee did have a decision to make, then you’re on a wing and a prayer [after he does].
"You can look at it two ways, his studs are up onto the ball, then his foot hits it [the ball], and then he hits the Cardiff boy quite high. I’ve seen them given, seen them not given. But that's not what cost us today, it is that first goal."
Coleman had admitted in the run up to the game that he may not pick Ndong given intense speculation over his future this week.
Sunderland have rejected a loan bid from Watford for the 23-year-old and the Black Cats boss has accepted his departure is inevitable somewhere along the line.
Ndong is one of a number of players who wishes to leave the club but Coleman said his attitude in the game was good and that he would pick wantaway players if they were mentally right for the task.
He said: "We spoke with him yesterday, we know he is on his way eventually, but I said to players who want to leave, if they’re mentally with us and will put a shift in for the team we’ll use them.
"To be fair to him in the first half he covered a lot of ground, worked hard. He got sent off, maybe he is unlucky.
"I really can’t say Didier Ndong was the problem today. We gave the opposition a goal 45 seconds into the half, make no mistake that is where it came from today. Then it’s ‘here we go again’."