Sing featured off the bench in England’s opening win against the United States in Sunderland and played in the record victory over Samoa.
Her dead-eye kicking for the posts is a big strength, but that is unlikely to be needed against Scotland given the form of fly-half Zoe Harrison.
Kildunne and Sing are the two specialist full-backs in the squad, but with only 12 caps, the Gloucester-Hartpury kicker remains inexperienced at the highest level.
Dropping in at first-receiver and being an extra playmaker is a skill that Sing has continued to develop under Mitchell, with her running game also more evident.
Utility back Helena Rowland was England’s starting full-back at the World Cup in 2022 before an injury ruled her out of the final, which Kildunne started and scored in.
An impressive record 27 points from fly-half against Samoa and a superb performance off the bench from full-back when Kildunne went off on Sunday might sway the decision in Rowland’s favour.
Covering multiple backline positions on the bench has been Rowland’s regular role under Mitchell, but with 43 caps and a World Cup semi-final start at full-back, the 25-year-old looks more polished than Sing for knockout rugby.
“If I was coach, I’d pick Helena Rowland,” former England wing Ugo Monye told the BBC’s Rugby Union Weekly. “We know that a high-performing team needs the spine of the team to perform really well.
“She’s one of the most understated world-class players at this tournament. The biggest compliment I can give her is she makes everyone look so much better. Everyone functions so much better around her.
“She’s so easy to work with and being so dominant at 15 it takes pressure off her wingers, whether that is Jess Breach, Abby Dow or Claudia Moloney-MacDonald.”
Former England fly-half Katy Daley-McLean also feels Rowland is the natural fit to replace Kildunne on Sunday.
Defensively, where Sing has sometimes struggled, Rowland thrived against Australia.
One such moment was during the second half when she made a superb tackle on Wallaroos’ wing Maya Stewart, despite being isolated out wide and up against a player with greater pace.
“England want to defend high which left her [Rowland] really vulnerable,” Daley-McLean added.
“I thought Helena was really good. Ellie won’t be playing so it gives them an opportunity to see Emma Sing in there, but I can’t see her featuring further in the semi-finals or the final.”
If Kildunne, who turned 26 on Monday, comes through her mandatory minimum 12-day stand-down period without showing further concussion symptoms, she will be fit for a potential semi-final on Saturday, 20 September.
However it pans out, Mitchell has options at his disposal.
