"Zak has been our best bowler who has always led the bowling attack, whatever the conditions may be, whether it is a flat track or a seaming track," Dhoni [ Images ] said on the eve of the first Test.
"He is always one bowler who has done really well. It is a blow, but at the same time we have to see the longer picture. We have to think about the World Cup also.
"We are so close to the World Cup, and any injury to any of your best 11 players may have a bigger impact, so that's also a bit of a concern. Till we are 100 per cent sure about his fitness and injury, we won't risk playing him," he added.
Zaheer is struggling with a hamstring injury and bowled only a few overs at the nets during the two training days. He did some fitness drills with team physio Paul Close on the first day and donned the goalkeeper's role in a warm-up football match the next day.
Dhoni said either Jaidev Unadkat or Umesh Yadav may earn their first Test cap if Zaheer happens to miss the opening Test.
"If Zaheer can't play then either Jaidev Unadkat or Umesh Yadav may make Test debut. Same is the case with number 6 batting spot. Pujara is pushing for that spot, but let's see. You have to wait till tomorrow morning," Dhoni said with a big smile on his face.
India [ Images ] didn't train on the eve of the match because it was raining in Johannesburg.
"We have had two big practice sessions and that is why we are ready for the big battle. Gary Kirsten has got lot of experience of playing in these conditions and his knowledge is going to help us," Dhoni said.
"Overall coming season is going to be exciting as Indian team is going to play world cup and away tours like West Indies [ Images ], England [ Images ] and Australia [ Images ]. Playing in different conditions is huge challenge," he added.
Talking about pitch and weather conditions, Dhoni said the strip is damp at the SuperSport Park and toss would play a crucial role tomorrow.
"We saw pitch from distance yesterday. It looked damp. Because of cloudy conditions pitch is going to remain like this. Thus toss will be vital," he said.
"Whoever wins the toss will get at least 60 per cent advantage of bowling 1st in the game. Not only pace and bounce but lateral movement is going to cause bit of worry for batters. I hope that our opening pair continues giving us good start," said the 29-year-old skipper.
Speaking about Sehwag he said, "There can be only one Sehwag. He never takes tension of the situation. He backs himself to play his natural game and that is why he is so successful."