On-loan Reading winger Henry suffered a hamstring strain during last week's 2-0 defeat at Gillingham, a game striker Alexander missed with a heel injury. Both remain doubtful starters but Jackett has not ruled either out.
There is also positive news on frontman Neil Harris, who will be fit despite limping off against the Gills with a calf injury.
But the Lions' treatment room remains crowded with Jason Price missing with a thigh problem and fellow forward Ashley Grimes out after pulling a hamstring playing for the reserves.
Defenders Zak Whitbread (hip), Paul Robinson (ankle), Tony Craig (back), Danny Senda (Achilles) and Pat O'Connor (cruciate) are also on the sidelines while George Friend's loan spell from Wolves has ended.
The match has been designated 'Tacheback Day' at The Den, with fans urged to sport a moustache to help raise awareness for male cancers.