Gordon picked up an ankle injury during the 4-4 draw with Luton Town last time out while Alexander Isak recently picked up a groin injury in the 3-1 win at Aston Villa.
Reflecting on the situation, Howe said: "[Gordon] hasn't trained this week. We don't think the injury is serious but we're hopeful he'll be back sooner rather than later. We're not sure [when that will be].
"Alex won't make this game but we hope he won't be too far away from the following games. We don't have a set return date but it's not as bad as first feared."
After Forest, Newcastle host AFC Bournemouth at St James' Park next weekend before travelling to Arsenal the week after. By that point, Howe is hoping to have the likes of Joe Willock and Elliot Anderson back in contention, though he couldn't confirm whether they would be back by the end of the month.
Willock has missed the majority of the season with an Achilles injury while Anderson has been out since October with a stress fracture in his back. "They're doing well at this moment in time," Howe added. "Joe is working really hard, he's not trained with the group yet but he's working really hard with the sports science team.
"He's in the the final stage of his rehab so we hope he'll be back very soon.
"Elliot is not far away either. He's really picked up his running so they're both getting closer."