The Scotland-born left-back has had a scan on a sore knee and is expected to be on the sidelines for two weeks.
Like Iredale, Jack Bridge (ankle) and Harry McKirdy (knee) also missed the 1-1 weekend draw at Morecambe and will be checked.
Loan pair Canice Carroll and Olufela Olomola could return after playing in the original 2-2 draw in Gloucestershire.
Forest Green should welcome back skipper Joseph Mills.
The former Southampton and Oldham defender missed the League Two home defeat to Scunthorpe on Saturday and was replaced by his brother Matt.
Elliott Frear and Dominic Beard will both be monitored, but Dayle Grubb (thigh) and George Williams (broken leg) remain sidelined.
Rovers boss Mark Cooper must decide whether to give Lewis Thomas another opportunity in goal or recall Jojo Wollacott.
Preview - Carlisle United v Forest Green Rovers
The Scotland-born left-back has had a scan on a sore knee and is expected to be on the sidelines for two weeks.
Like Iredale, Jack Bridge (ankle) and Harry McKirdy (knee) also missed the 1-1 weekend draw at Morecambe and will be checked.
Loan pair Canice Carroll and Olufela Olomola could return after playing in the original 2-2 draw in Gloucestershire.
Forest Green should welcome back skipper Joseph Mills.
The former Southampton and Oldham defender missed the League Two home defeat to Scunthorpe on Saturday and was replaced by his brother Matt.
Elliott Frear and Dominic Beard will both be monitored, but Dayle Grubb (thigh) and George Williams (broken leg) remain sidelined.
Rovers boss Mark Cooper must decide whether to give Lewis Thomas another opportunity in goal or recall Jojo Wollacott.
Head to Head Comparison
Carlisle United - Last 5 Matches
Forest Green Rovers - Last 5 Matches