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.
Coventry will be without defensive duo Ben Turner and Marcus Hall for the clash.
Centre-back Turner was forced out of Saturday's dramatic 2-2 draw against Middlesbrough with knee and ankle knocks while full-back Hall also picked up a knee problem.
Elliott Ward and Patrick van Aanholt were the two players to replace them and it remains to be seen whether manager Chris Coleman will go with the pair again.
Isaac Osbourne overcame a knee injury in time to face Boro and he will again be hoping to keep his place in midfield, despite coming off for Aron Gunnarsson at half-time.
Defender James McPake (shoulder) and midfielder David Bell (ankle) remain long-term absentees.
Sporting Life