22-year old midfielder Henderson is available on a free after he was released by Arsenal at the end of last season while 20-year old Australian Fletcher, who played under Bruce at Sunderland, is also without a club.
"There's more to come from him. He scored a free-kick and played some lovely passes around the pitch so we've been impressed. He's someone we'll look to try and do something with to get him in on a permanent basis.
"I think he's come on and improved, so he could be someone that can help the group."