"We should have been 4-0 up before they scored," said Warnock.
"We hit the post three times in the first half and missed an even better chance than any of those but I was disappointed with the way we lost in the second half.
"The way we played after the interval told me a lot about my players - some good and some bad.
"It's a great challenge for me to sort it out and have a go next year. We haven't got much money to spend but the chairman is very supportive and it could be an interesting season.
"You have to give some credit to Doncaster. It's a fantastic achievement for them to come new into the league and finish around mid-table when they were bottom at one stage. They are a lovely club and have some lovely fans."
Doncaster manager Sean O'Driscoll said: "I think we've shown we can compete in this division now.
"We had a bit of luck when they kept hitting the post but against that we should have had a penalty and when the chances came we took them.
"The supporters have helped us a lot this season because they have stuck with us when results weren't going our way.
"We were bottom of the table at Christmas and I'm proud of the way we've managed to climb up the table while at the same time sticking to playing football the way we have always done."