On a day when the Reds could have moved seven points clear of the side just outside of the Championship top six in Reading, Barnsley turned in arugably their worst display of the Ismael era to lose out to lowly Coventry, managed by ex-Oakwell chief Mark Robins.
A goal from Dom Hyam put the struggling Sky Blues in front on nine minutes and Barnsley - who came into the game on a club record run of six straight away league wins - created little by way of a response.
The win was sealed in stoppage-time by substitute Matty Golden, who looked suspiciously offside. But there was no doubting the merit of victory for Coventry, who took a giant step towards Championship safety.
Barnsley now move onto successive Yorkshire derbies against Huddersfield Town and Rotherham United as the sixth-placed side bid to get another bad day at Coventry - Barnsley have not beaten a Sky Blues side at their home since 1923 - out of their system at the earliest juncture.
Ismael said: "I think it was not enough to win. We were sloppy in the first half and we were not really there and there was no intensity. We conceded the early goal and after, it was easier for Coventry to defend.
"We did not have a corner in the first half and only in the last second did we have a chance from Cauley Woodrow. We tried to push and have more pressure, but we did not really shoot for the goal. Now we are close to the end and have four games left and cannot complain or try to analyse the game.
"It is all about getting the next game and focusing on Huddersfield and it is all in our hands and we are still in the perfect position and now the focus is on the next week.
"Coventry changed the way they play and you have to adapt and they deserved to win. It was enough for us to pretend for the three points and 80 per cent will not be enough. Maybe it is a good warning to raise our intensity and desire to get the points.
"We took too many touches and we were too late and the intensity was not there and you have to lift it all the time to stay in the game and put the pressure on the opponent.
"My feeling was there that we would not score and we have to accept it was a bad day and now we focus on Wednesday's game.
"We are not over the line and it was a reminder for everyone. The next game is Wednesday and our purpose is to bounce back. "