Sullivan, 43, was in inspired form throughout the promotion clash making a number of expert saves to keep Doncaster top of npower League One.
The former Tottenham, Leeds and Chelsea goalkeeper only arrived back at his parent club on Friday after a loan spell at AFC Wimbledon was cut short due to an injury to regular number one Gary Woods.
Flynn said: "Bringing Neil back was a big boost and he was absolutely magnificent.
"It was like having a young Neil Sullivan back and he played a big part in the result.
"It is almost worth a goal having a keeper like that. He made a great save in the first half and showed his experience by getting in good positions - it was exactly what we wanted.
"We played a good team, who gave us serious problems all over the pitch. I think it is a big win for us at this stage of the season.
"It is a big win against one of the big teams in the competition - it was absolutely vital."
In a hard-fought contest, the Cherries started brightly and almost took the lead when skipper Steve Cook rattled the crossbar with a header after five minutes.
Sullivan then won his battle with opposition striker Brett Pitman, tipping a close-range header over the crossbar and diving low to his right to keep out a fierce drive.