Goals from Alex Tabb and DJ Campbell sealed the win, but Allen was not convinced with his side's display - starting with the trip to Chesterfield this week.
He said: "We must prepare for Wednesday with a single-minded desire to put that first game behind us, and to improve.
"I have watched the Scunthorpe game again on video - and we have plenty to work on."
He said: "We must prepare for Wednesday with a single-minded desire to put that first game behind us, and to improve.
"I have watched the Scunthorpe game again on video - and we have plenty to work on."