O'Neil was called into action three minutes before the interval when Digard was forced off the field with a groin problem.
Digard is due to undergo scans later in the week which will determine the extent of his injury but could now be facing a spell on the sidelines.
Gareth Southgate: "Gary was due to have a hernia op on Tuesday, but he is now going to play through it for a little while.
"He's been suffering some pain so that's why we didn't start him against Sheffield United, but he feels a little better after Friday.
"Didier has a groin injury, possibly a small tear, we will know more on Wednesday when he has scans.
"Given the injury to Didier, Gary wants to try and help the team by remaining available, so the plan now is for him to have the op just before our game with Bristol City (on August 29)."