Ellington is suspended after being sent off towards the end of Saturday's 1-1 draw at Reading and Helguson is still sidelined with the calf injury he suffered against Leicester the previous weekend.
Danny Graham, who hit his sixth goal of the season at the Madejski Stadium, will continue as the lone frontman.
Skipper Jay DeMerit will be checked over having missed the last four matches with a groin injury and then an eye infection.
Adrian Mariappa will continue at centre-half with 17-year-old Lee Hodson staying at right-back if DeMerit is ruled out again.
Midfielder Scott Severin remains doubtful with a groin injury.