The midfielder was recalled from his loan at the beginning of the January market and impressed in last week's win against Aston Villa.
Ahead of tonight's clash with Leeds, Moyes said: “Harrison had his best game for us (against Aston Villa). I thought that he had an impact in the game which he hadn't had really in the other games, but neither did the team, because we hadn't performed.
"We played better against Aston Villa than we had in the other games but I was impressed by Harrison, he did well.”
However, with Idrissa Gueye back and the medical room clearing, Moyes says Armstrong could yet be sent back to Deepdale.
Moyes said: “We'll see what comes around but I think the big thing for me is Harrison (might) get a chance to probably play another 20 games for a club that are probably going to be in the play-offs in the Championship.
“That for a 19-year-old is really, really big and important. If you look at the player's development, we've got to watch that we don't make the wrong decision because we are saying ‘let's have him around here’.
“I don't really want him sitting on the bench for me when I think he could be playing.”