Danny Coles was shown his second red card of the season in the 4-0 defeat at Carlisle and will face a four-match ban, meaning that Penney's top priority is to strengthen at the back.
Speaking to the Evening Post, Penney said: "There will be at least one new signing and we will keep trying to add to the squad.
"My first priority has always been defenders as I like to build from the back, but I would like to bring in a defender, a midfielder and a striker.
"There's not enough experience in the side to make sure we don't get caught chasing back.
"I know now why so many goals have been conceded and there's a lot of food for thought."