Gordon was withdrawn in the first half of Wednesday's win over Athletic Club after falling to the floor holding his hip following a shot at goal.
Howe confirmed after the game that the issue appeared to be "a recurrence of the hip injury that has been plaguing him in recent weeks."
The winger has appeared short of full fitness for some time and remains without a goal or assist in the Premier League this season.
Asked for an update on Friday, ahead of the domestic action, Howe stopped short of completely ruling Gordon out of facing Brentford, but admitted it was unlikely he would play.
Howe told reporters: "We don't think it's a serious injury. We're going to get him scanned, and we're awaiting the results.
"There's every chance he might miss out on Sunday but we'll give him every opportunity to be fit."
Summer signing [Yoane Wissa](/topic/yoane-wissa) is in the final stages of his rehabilitation from a knee injury, suffered days after joining Newcastle for £55million.
Howe says Wissa is building up his fitness and suggested it may still be some time before the striker can make his debut.
"He's working well," Howe said of Wissa. "He's on the grass and he's building his fitness. The knee is feeling good and there are no problems with that.
“It's more a case of looking at his fitness, sharpness, all the things he's going to need to put together.
He's in a good place to kick on and push his body. We need to give him time to get robust enough and be in a position to give his best for the team. I'm sure he'll be worth the wait."