Dave Jones said: "Sometimes you get hammered for the substitutions you make so it was nice to get it right.
"I am just pleased for Jason and so are the rest of the players. He's worked really hard to get back and it was a great free-kick to put us back in front.
"In fairness to the goalkeeper, we had Craig Bellamy, Peter Whittingham and Jason over the ball and I had no idea myself which one of them was going to take it."
City rallied late on after seeing their first half lead wiped out with 12 minutes to go. This time though Cardiff came fighting back and took all three points with two Koumas goals in the final minutes.
Jones added: "I felt we were in control of the game at 1-0 but we just switched off for a spell and allowed them to get a bit of momentum.
"We had to step it up after they equalised and I felt we came away with a deserved victory in the end."
Defeat for Doncaster leaves them seven points and two places away from the relegation zone.
Manager Sean O'Driscoll could not hide his disappointment at the loss, saying: "That's three games in a row where we have played well and got nothing to show for it.
"Obviously it's disappointing and frustrating but we have to lift ourselves now for Preston at home on Tuesday night.
"It felt a bit surreal at the end of the game because with five minutes left I thought we might go on and win the match.
"There was a period midway through the second half where I thought they were out on their feet but then when we walk off at the finish we are 3-1 down.
"I thought our gameplan worked and if you look at the stats we were probably the dominant team.
"But they are a talented side and you have to give them credit at the end of the day."