Paddy Wallace is expecting to return in the centre after sitting out last Sunday's league win in Llanelli, meaning Marshall or Darren Cave will miss out.
It looks like being a straight shootout between Gilroy and Andrew Trimble on the left wing, with both players scoring tries in recent weeks.
Meanwhile, Roger Wilson's man-of-the-match display against the Scarlets should see him hold onto the number 8 shirt for the meeting of Pool 4's top two sides.
That would leave Nick Williams contesting the blindside flanker role with Henderson, who started the opening round games against Castres Olympique and Glasgow Warriors.
"There are some tough decisions to make, but this is what a coach wants," insisted Mark Anscombe, who has guided the Ulstermen to 12 straight wins across two competitions.
"We talk about players performing and forcing our hands for selection, which in turn makes it easy for us.
"The players have already been told who is playing and the ones that have missed out have been outstandingly positive. In order to survive you need good internal competition through a long season. You certainly cannot survive on 15 players alone."
Both Anscombe and former Saint Wilson know their round 3 hosts are certain to lay down the gauntlet in some specialist areas, most notably the set pieces.
"Northampton's set piece is very strong and they just love scrummaging. They get a lot of penalties in that area and they also have a big maul from the lineout," explained the New Zealander.
"We have to be excited about meeting that challenge head on and not hiding from it. Basically we have to hold that and conquer it."
The basis for Ulster's defeat of the Scarlets was a strong set piece with Callum Black seizing the opportunity to impress at loosehead prop, where he is providing stiff competition for Tom Court.
Although he has had an injury-interrupted start to the season, Wilson seems to be coming to the boil at just the right time as Ulster prepare for a busy December period.
"Yeah, I'm happy so far (that I made the right decision in coming back to Ulster). It's early on in the season, I wouldn't say a game like this you could say you've made the right decision or not," he told BBC Northern Ireland.
"When you come towards the end of the season and you're playing for silverware then you can make the judgement whether you've made the right call or not.
"I chose to come back. I had a fantastic time in Northampton, four great years and I really enjoyed it - absolutely no regrets whatsoever.
"Now I'm part of a team which is being talked about as one of the top teams in Europe, a team that can actually compete to win stuff. So far we've done that, so far we haven't lost a game and hopefully we can keep that going."
The 31-year-old knows that following a series of erratic yet winning performances, Ulster will need to hit top gear if they are to get a result this weekend.
Bracing his Ulster colleagues for a sharp 'step up in intensity', Wilson remarked: "It will be a strange experience coming up against my former team, but once I'm on the pitch it will be down to business.
"Northampton have operated the same game-plan over the past four or five years - it's basic but what they do, they do it very well.
"Their key weapons are their set pieces - particularly their scrum and driving maul - very few teams can stop their forward power.
"They are a formidable side. We've done well so far this season, the last few weeks have been a little bit stuttery though and the step up in intensity will be huge against Northampton. We have to be ready for that."
The Ulstermen currently hold a three-point lead over Northampton at the top of the pool, with third-placed Castres still in touch on four points.
Anscombe is hoping Ulster can negate the effects of the short five-day turnaround between the Scarlets and Northampton games, and he has been delighted with how the players have approached it.
"It's tough but we have known about it for some time so there are no excuses. We've just got on with it," he explained.
"The boys were back on duty on Monday, despite a problem with a delayed flight back home. I didn't hear any gripes since despite some getting to their beds by 3am. To me that shows the guys are focused on the job in hand and not looking for excuses."