Bentley, who came in for the injured Paul Smith, saved spot kicks from both David Worrall and Mark Hughes, to help the Shrimpers advance 3-2 on penalties to the next round after the game had ended 1-1 at Roots Hall on Tuesday night.
And Sturrock insists Bentley is now pushing to become the club's number one keeper.
"Daniel was terrific out there and made some fantastic saves both in the game and in the penalty shoot-out," said Sturrock.
"He's shown that he is really challenging Paul Smith to be my number one choice and he's done well whenever he's been called upon this season.
"At the start of the campaign if my first choice keeper had been injured then I would have been concerned but Daniel has gone up in my estimation now and we've seen how much he has improved.
"It's great to see and I'm very pleased to have won the tie.
"We did the ugly side of things very well and I thought we just about shaded it."