The sixth-placed Reds can push themselves to the brink of Championship play-off qualification with victory in their penultimate regular home match of the season, while a defeat would represent another significant blow in Rotherham’s survival quest at the other end of the table.
A home victory would put Barnsley nine points clear of the side just below them in Reading, who are not in action until Sunday lunch-time when they host Swansea City - their 44th league game of the season.
Should Ismael’s side beat the Millers on Saturday, their top-six berth would be sealed if the Royals drop points against the Swans.
Issuing a warning, Ismael - whose side have been troubled by two strugglers on home soil in recent times in Sheffield Wednesday and Derby, said: “We need to stay focused, we are not over the line.
"It will be a tough game against Rotherham, they are fighting to survive and will give everything on the pitch and my expectation is that it will be a very intense game about duels and second balls and we have to be strong in that area and do the basics and be ruthless to get the three points.”
Admitting the game will be totally different to the match at Huddersfield, he added: “One hundred per cent, it will be more like Millwall and Birmingham, this is my expectation and we need another performance and way to play football to win the game.
“I like the mentality and the attitude of the manager (Rotherham United manager Paul Warne) and the way they play because they have one way to play and it is exactly the same and they have focus on their own principle.
“They are like Wycombe and have a ‘never-give-up’ mentality and give everything in every game and you have to respect that.
“It will be intense and Rotherham have to fight and we have to match that.”
Meanwhile, Ismael says that forward Dominik Frieser could be out for two to three weeks with a hamstring issue, but revealed that Herbie Kane (knee) could return to training next week.
On Frieser, he said: “We will see. It could be a couple of weeks, maybe two or three. We will see. I cannot say anything. I know last year he had the same injury and came back quickly. If it is the same case, it is good for us.
“If not, we have to accept that and find the other solution. We have enough players in the squad at the minute.
“You are never happy as a manager, but you have to deal with it. You have to take into account that something can happen, but we have the solutions. After a long season, it is normal that you have to accept it sometimes.
“We try everything, but at the end of the day, the tiredness and fatigue from all the season comes out and you have to deal with it.
“Herbie Kane, if everything goes right, will be back next week training with the team. Liam Kitching is reacting very well with the training session with the team. Ben Williams has his own programme and is making small steps forward, which is important.”