Wagstaff played a major role in helping Charlton to promotion last season, but has made just two appearances this term and had a spell on loan at Leyton Orient.
However, he marked his first Championship start of the campaign with Charlton's crucial second goal at The Valley on Saturday.
Powell said: "I'm very pleased for him. He's a good young professional who has been out on loan and has bided his time.
"He's always been looking to get a chance but has trained ever so well, especially this week. We worked on one or two things, but I never told him where I was going to play him until the morning.
"I'm pleased for him as he's a Charlton boy and he got a very good reception from the supporters and he will always give you that 100 per cent endeavour.
"He's got quality as well, and for him to come in on the left and play with Cedric Evina down that side, I'm really pleased he got his goal and put in a really good performance for 75 minutes."