The skipper has not featured since limping out early on in the first FA Cup tie against Liverpool at Montgomery Waters Meadow late last month.
Norburn collided with the Reds' Curtis Jones early on before limping off 10 minutes later and the bang to his knee has proved more problematic than initially feared.
The influential midfielder is now unlikely to make another appearance in the three months left this season as injuries continue to bite Ricketts and his squad, which the manager has described as 'bare bones'.
"Norbs isn't good, that's going to be a while before he's back unfortunately," said Ricketts. "We're writing off our best player and that's disappointing."
Asked if he is unlikely to feature again this season the boss added: "Yes and there's nothing we can do about it.
"It's very easy to gloss over the number of games we've had, what the FA Cup run does to the team emotionally and physically, easy to gloss over injuries, but we are coping with all these things which aren't going our way."
Shaun Whalley and Brad Walker are also missing with hamstring injuries while Josh Vela is being managed to ensure he is in the best place physically to make his debut, which could be soon – although not at Portsmouth on Saturday.
"Shaun may come back in the next couple of weeks or so," Ricketts added. "We're all hands on deck at the minute, we are what we are," he added.
"If you're fit you're in the squad. We're light on certain areas but there's nothing we can do."