The England international has struggled for form in the Premier League of late and was forced off in the first half of Wednesday night's Champions League win over Athletic Club.
Howe says it's too early to know the severity of the injury, which will be assessed in the coming days, but Gordon is now a major doubt for the weekend trip to Brentford and England's World Cup qualifiers this month.
"I've not spoken to him and I haven't had the full rundown from the medical team," said Howe.
"It seems like a recurrence of the hip injury that's been plaguing him in recent weeks.
"We'll assess that and see what damage is done."
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Will Osula will also be closely assessed in the coming days after Newcastle opted to withdraw the striker from the squad on Wednesday night during the warm-up.
Osula has been struggling with an ankle problem for weeks now and looks set for a spell on the sidelines as the Magpies look to get to the bottom of the issue.
"We've been in discussions with him at length about his ankle in the last few days as to whether he's fit enough to perform to the level we need and he's comfortable with," said Howe.
"After a discussion we felt it was better to withdraw him and get his ankle 100% right."
Newcastle were boosted by the availability of Bruno Guimaraes for Wednesday night's game. The captain missed training though illness on Tuesday but played on Wednesday night, even though he admitted afterwards it felt like he had "10 kilograms on his back".
In response, Howe laughed: "He wasn't saying that to me this morning, he said he was feeling a million dollars!
"In all seriousness his character is undoubted and his will to play is so strong.
"I did have a detailed talk with him and he assured me he was fit to produce a performance that the team needed while probably acknowledging he wasn't at 100%.
"That's why we withdrew him when we did, to try and protect him. But I think we lost a little bit when he came off the pitch speaking honestly. That's why he's so important to us."
Lewis Hall returned from the bench for his first taste of the action in more than a month.
Howe said: "It was my plan to try and give him minutes. You can never guarantee but it was in my mind to use him.
"I didn't intend to use him at West Ham because he's only had one training session and he's worked so hard to get back.
"We didn't want to lose him having done all the hard work.
"It was the right time to introduce him. I thought he played very well, I was really pleased. We still have to have a level of caution to how much he can play, but positive steps for him."