Mohun Bagan are winless from the last three matches and are three points behind reigning champions Begaluru FC (35).
However, to their advantage, Mohun Bagan have one match in hand.
The green and maroon brigade are also in search of an elusive title since winning the Federation Cup in 2008.
After taking on the Shillong outfit, Mohun Bagan will face Sporting Clube de Goa (May 23) in the penultimate round before concluding their campaign against Bengaluru (May 31).
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Goalkeeper Debjit Majumder, who has played a major role in the team's campaign, is ruled out because of an injury.
"We have spotted a blood clot between his shinbone and knee. He is out of tomorrow's tie," Shankarlal Chakraborty, the assistant coach of the team, said.
Denson Devadas continues to be sidelined with an injury, while Kinshuk Debnath is expected to start in place of Anwar Ali.
The team will be dependent on the likes of Pierre Boya, Sony Norde and Balwant Singh to score.
The Royals are ready to make their Kolkata debut after a long 17-day break. Santosh Kashyap, the coach of the team, will try to end their fairytale journey on a high note against his old club.
"Finishing second or third in our debut season will be a major achievement. We had a long break and we will go for the kill tomorrow," the coach told the reporters.
The former ONGC coach will have a full squad to choose from unlike his Bagan counterpart.