The 27-year-old contributed 24 goals and 13 assists last season for the Saints as they clinched promotion to England’s top division. Since then, the striker's appearances have been limited and in the chances he has earnt he has failed to show his worth.
He started the first three Premier League matches but was removed during the Brentford game and has only featured off the bench since. Martin spoke to the Daily Echo about the forward's poor start and how he just needs to continue working hard.
"He just needs to preserve and keep working. Arma's really frustrated at the moment, as he should be," said Martin.
"It's probably been a bit hard on him because we signed Cameron Archer and Ross Stewart had come back fit. Arma played predominantly on the right for us last season, pretty much all season. Ben Brereton Diaz plays wide left, Arma plays wide right.
"Paul Onuachu and Ross are very similar profiles so in case you need them at some point and then you have Cam. It's like they're all very different. Arma just needs to be patient and trust that we absolutely love him still and we know what he can do. There'll be games where we really, really need him."
Martin admits that he should have given Armstrong more starts this season and said that if his work rate continues, he will play a huge role in keeping the club away from relegation this season.
"And looking back, I should have put him on the pitch on Monday night at Bournemouth so it's my fault, not his.
"I've told him that and he hopefully accepts that. Whilst being frustrated, he trained brilliantly and I know he's ready for us whenever we need him.
"He'll have a big part to play. It's been a bit of a frustrating few games for him but that's football. It's never linear. It's never a straight line.
"He needs to stick with it, keep working hard as he has done in training and use the frustration he feels when he gets his opportunity. I've got no doubt he will."