The 29-year-old signed for the Blues on transfer deadline day.
He arrived on loan from Nottingham Forest until the rest of the season.
It’s the Irishman’s second stint at Fratton Park, having made 27 appearances during the 2011-12 Championship season, and he’s set to replace Luke McGee in between the posts for Saturday’s clash against Doncaster.
Pompey boss Kenny Jackett said: ‘I would think he would come straight in (to the starting line-up).
‘I never neccesarily pick the side on a Thursday, but for Luke I feel a step backwards to go forwards in the long run will be good for him.
‘Stephen Henderson is someone I have been interested for a while and I’m pleased to get him.’