In order for Bangladesh to qualify for the semi-finals of the Asia Cup, several equations have to come in Bangladesh's favor at the moment. First of all Sri Lanka won against Bangladesh by a big margin so Sri Lanka is tension free for now and Sri Lanka will definitely try to play the match against Afghanistan free. Bangladesh must win by a big margin against Afghanistan to qualify for the semi-finals because if Bangladesh can't win against Afghanistan by a big margin and if Afghanistan can beat Sri Lanka in their last match then Bangladesh's dream of qualifying for the semi-finals will end so Bangladesh must win against Afghanistan. Besides, the dream of going to the next round can be fulfilled only if the victory has to be secured by a big margin.
Bangladesh did not perform well against Afghanistan in recent times. Bangladeshi batters always struggle against Afghan spinners. Especially Mujib and Rashid Khan. They have quality pace bowlers as well. But the problem is Bangladesh's batting line up. They are missing some good batters at this moment and playing with some young players.
Bangladesh has to do its best in all three sides to beat Afghanistan. But as I said, they did not perform well against them recently and it won't be easy for Bangladesh to beat them. I cannot imagine about the margin at this moment.