"It's a big achievement but nothing is more important that (Sunday's) match," said Mortaza, 35, who will stand in Bangladesh's upcoming 11th national parliamentary election from his constituency, Narail-2, on Dec. 30. "(Sunday's) match is very important. We must win it if we want to gain the momentum for the whole series."
The captain said it has been a relief for Bangladesh to have Shakib Al Hasan and Tamim Iqbal back for the series.
Both Bangladesh stalwarts were injured during the Asia Cup in the United Arab Emirates in September, and although Shakib returned for the test series against West Indies, it would be the first match since for Tamim.
"Obviously, Shakib and Tamim being in the team is a big advantage," Mortaza said. "The fact that Tamim played the practice match and made runs is a big relief for us and him as well."
Tamim's return left Bangladesh to decide its top order combination because of other players' form.
"Having an in-form top order is a sweet problem to have," Mortaza said. "It might be the case that one or more of them (could bat lower down the order). Nothing is for certain."
West Indies has not won a bilateral ODI series since its victory against Bangladesh at home in 2014, but captain Rovman Powell is confident his team will turn it around.
"We haven't won an ODI series in a long time," Powell said. "The guys are upbeat and raring to go. No better opportunity to change it in Bangladesh. We have played some lovely cricket in India but we didn't get the results that we wanted."
Powell believes winning the key moment of the game will be important in deciding the series.
"It will be very equal," he said. "Both teams have good spinners, fast bowlers and batters. It will be a very competitive series, and it will come down to who wins those key moments in each game."