The midfielder missed the Addicks' opening-day defeat at Bournemouth with a calf injury and then had to come off at half-time in Saturday's 1-0 reverse against Middlesbrough.
"He got a calf strain in training 10 or 12 days ago," said Powell.
"We thought he would be right (to face Middlesbrough) but you can't replicate what happens in games.
"The pressure told and he didn't make it. It's a shame and we have to get him fit, he's inspirational for us.
"He was our joint top scorer last season and is a vital player for me, Hopefully he will be okay."