The Sudanese winger, who joined the club from Scunthorpe United in the summer, has played just once for City since joining the club - in the Carabao Cup tie at Nottingham Forest.
He has been affected by a groin injury and while it is initially expected that he would be out for a few weeks, a scan has revealed that he faces a more longer period out.
It follows the earlier news regarding Angol (hamstring).
Adams, whose side welcome Walsall on Saturday, said: "We are looking at a period of time where both are going to be out for a longer period that we had hoped for.
"I would hope that he (Eisa) will be shorter than Lee and I hope that Lee will be shorter than the three months (previously reported).
"He (Eisa) had another scan and it is a bit longer. But we are hopeful it is not as long as everyone thinks. But you just never know with these injuries, in how long it is going to take."
Confirming that Oscar Threlkeld and Levi Sutton are both fine for the weekend's game after their recent spells out with injury, Adams added: "They are fine and okay and back in full training. Yann Songo'o has obviously (also) played the last two games and so we are back in relatively good shape.
"Levi is fine and ready to go and came on the other night and was very competitive and covered the ground very well to make a couple of tackles and I am delighted.
Meanwhile, Adams says that it is unlikely that deadline-day signing Theo Robinson will feature this weekend, due to a lack of match-fitness.
The former Port Vale player, signed on a free transfer on a deal for the rest of the season, has not started a competitive game since May.
"He is with the medical department today and until they have finished and released him, we will wait and find out from them, so I would think it would be highly unlikely he will be involved on Saturday," Adams continued.
The trio of Kian Scales, Jorge Sikora and Reece Staunton will be allowed to move out on loan to a National League club, ahead of their transfer deadline, Adams also stated.