Soumya was not part of the original squad announced by the Bangladesh Cricket Board (BCB), but has now been drafted in as cover amid concerns over Litton's fitness and opener Parvez Hossain Emon, who is currently suffering from a fever.
Explaining the decision, Litton said the team did not want to take any unnecessary risks ahead of the three-match series.
"The reason Soumya was included is because I was fifty-fifty. We don't know what might happen and we'll have to wait until tomorrow. Emon also has a minor illness, so we didn't want to take the risk of suddenly calling someone in at the last moment," Litton told reporters.
"We tried to keep Soumya with the squad. He may not be able to train today, but from tomorrow he will be fully involved with the team."
Litton sustained the injury during the third ODI against Australia in Mirpur, where he briefly retired hurt before returning to score an unbeaten 58 off 78 balls and register his first ODI fifty at the venue. Although the team physio had expressed concern after the match, the wicketkeeper-batter said his condition had improved.
"Firstly, I'm feeling better than before. I'm in a much better position than I was. But we'll assess things today and then decide tomorrow whether I'll play," he said.
Bangladesh enter the T20I series high on confidence after securing their first-ever ODI series victory over Australia, winning the three-match contest 2-1.
"It's definitely a positive. We didn't just win the series; we played very well in all three matches," Litton added. "Of course, T20 cricket is different and both teams are different, but most of our players were part of the ODI squad. There's confidence within the group, and on any given day you have to play very well."
The first T20I of the three-match series will be played in Chattogram on Wednesday.
