Al Nasr could also be without their star foreign striker Luca Toni, who is still carrying an injury.
Diane, who has scored all four of Nasr's goals in the Champions League, missed the Pro League match against Al Wasl last week with a foot injury.
Toni looked good on his return in that game after almost a month out with an elbow injury, but the Italian's left hand was heavily bandaged.
Nasr must win to keep alive their hopes of progressing in the competition and Walter Zenga, their coach, was putting on a brave face.
"I don't want to say that because of injuries, or because some players will not play, we have lost the game," Zenga said.
"We got 20 good players and we have been preparing well. I am not like other coaches who cry when they lose a game because of injuries to one or two players."
Nasr are bottom of their group with three points from four games, their only win coming against the two-time reigning Qatari champions Lekhwiya, who are third with six points.
Al Ahli, who missed out on the Saudi league title by two points this season, are top with seven points but only head Sepahan, who are in touching distance of their third consecutive Iranian league title, on goal difference.
To progress from the group, Nasr have to beat Sepahan, get three points in Qatar against Lekhwiya on May 15 and then hope the other results go in their favour.
"There is only one thing I want to say: that we are going to play to win the match," Zenga said.
"We have only one chance and that is to win this game."
Mark Bresciano, Nasr's Australian midfielder, has urged his teammates to show the same spirit they did on their Champions League debut in Isfahan, when they frustrated Sepahan until the 84th minute before a Bruno Cesar goal gave the hosts three points.
"The first game we played against Sepahan in Iran, we saw that they are a very strong team," Bresciano said.
"They are probably one of the favourites for the group and also to win the Champions League.
"But we demonstrated we could compete against these teams. We did hold off for 85 minutes there; they scored in the last five or six minutes of the game.
"But for us, this is probably the most important game because this is the last chance we have. It is a must-win game for us if we want to continue in this competition and have a chance of qualifying for the next phase.
"Without a win, we can consider ourselves out. So if there is going to be a future in the Asian Champions League for Al Nasr this year, it all depends on this game."
Sepahan are taking nothing for granted after their last game against Nasr.
However, the Iranian heavyweights, who have a two-point lead over Esteghlal in their domestic league, are determined to get the three points that should seal their place in the second round of the tournament.
"We respect Al Nasr and they gave us plenty of problems in Isfahan," said Zlatko Krancjar, Sepahan's Croatian manager.
"They have a very good coach and players, so I expect a very tough match.
"But are going into this game with a lot of motivation and the desire to win.
"We have seven points and our future is in our hands.
"Even if we lose this game, we have one match remaining with Ahli and we can get three points from that game and qualify."