The severity of Bentley's injury is not yet known, but he could be facing a lengthy spell out, leaving first-choice pair Garry Richards and Simon King as the only senior centre-backs available.
Stimson said: "We know it's an area we need someone else to compete in, unfortunately Mark Bentley's reported for training injured.
"He's going to have a scan on Sunday, so we won't know until probably the middle of next week, so we're even lighter in that position.
"We know it's a position we want to cover, we've been trying throughout the summer but we want to bring players in who are as good as what we've got.
"We've been quite fortunate that the two boys who played their last season have done us a good job so it wouldn't be worth bringing someone in to make the numbers up. I'm sure we'll have someone by the start of the season."