With head coach Sanjoy Sen missing th tie for personal reasons, assistant coach Shankarlal Chakraborty will take charge of the Kolkat giant in the crucial encounter.
The second leg will be played at the Rabindra Sarovar Stadium on next Tuesday.
Not just Sen, Mohun Bagan will also miss their important players like Sony Norde, Debjit Majumder, Pritam Kotal, Pronay Halder, Souvik Chakraborty and Jeje Lalpekhlua as they have rested for the tie.
Mohun Bagan thrashed Sri Lankan outfit Colombo FC 4-2 over two legs in the preliminary stage of the competition.
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Club Valencia, on the other hand, would be eager to return to winning ways after having lost all six of their last six league games which ended in December.
"Valencia are a very strong side and they have very good players. We have come here to win the game and we are fully prepared. For us every player is the same. Each player in our squad has the quality to play in the first team so it's not an issue for me," Mohun Bagan's assistant coach Chakraborty said on the eve of the match.