The national selectors will meet here tomorrow to pick the squad for the series.
Zaheer walked out just as he was about to bowl his last delivery of the innings' 99th over. Railways were 333 for six at that point.
It was the first over of Zaheer's fourth spell, and his 12th of the day. He did not return to the field thereafter.
Mumbai skipper Ajit Agarkar said that the senior bowler will turn up tomorrow.
"Zak is okay. He is just cramping a bit. He has had a long day and was it very humid here," Agarkar told reporters after the match.
Agarkar said that the veteran started feeling uneasy after tea and added that they didn't want to exert him as two wickets had already fallen in the session.
"He has worked hard enough. In the last month he trained well. He should be on field tomorrow morning. He should be okay. He is just cramping badly. It suddenly got a bit humid, so he probably dehydrated... After the tea session. That is when, that is when you want to hold back a little bit instead of doing any damage. He should be okay tomorrow morning.
"Once you are in (the dressing room) you can't bowl for that period so it didn't really make sense, plus they lost two wickets later on so they were eight down anyway. It's better when you are fresh tomorrow than making it worse today. That is the only reason." More