A hugely controversial result saw the hosts take the lead after just two minutes when Carlton Morris challenged visiting keeper Viktor Johansson in the air and netted the only goal of the game.
Replays showed that Morris caught Johansson with a leading arm and the Swede sustained eye socket damage in the incident and ended up being replaced after lengthy treatment.
Despite that, the goal - to the amazement of many - stood and despite dominating much of the rest of the game, Rotherham could not respond thanks to a mixture of poor finishing and inspired goalkeeping.
The win will secure a top-six finish for Barnsley, if Reading drop points against Swansea on Sunday lunch-time.
Ismael remarked: “It was a really, really poor performance. We got the three points and that is the most important thing. Big congratulations to Rotherham, they deserved more than to lose the game.
“It was the expected fight. Maybe we scored too early and woke up Rotherham, but we never came into the game. We were always too late and our decision-making was too slow.
“But at the end of the day, nobody will speak about the performance and it is just all about the three points and we did it. We made our job.
“Now it is just ‘wait and see.’
“In a season when everything goes in the right direction, you win such a game. If it goes in a bad direction, you lose such a game. This season has gone in the right direction for us and we take the three points.
On the hugely controversial goal, he added: “I did not see the situation from my angle. I have to see. At the end of the day, it does not matter now, we have the three points and I want to have an overview about all the season so far.
“I think how the boys have performed with 77 points is amazing. We performed so well in such a big game on Wednesday and we know we can do better and are able to have more influence and impact on the game. It is an amazing result to see what we have achieved so far.
“I have learned a new word ‘so near.’ But we are not over the line and we can wait for tomorrow and we will see. The guys’ job was to win the game and have a massive gap and now we will see if it is enough or if we have to get the next points.”