Eddie Howe, speaking after the 4-0 defeat to Manchester City, feared injuries to Fernandez and Lewis were “longer-term”.
Dummett, meanwhile, hasn’t kicked a ball this campaign due to a calf injury.
Before the visit of Manchester United however, Howe provided a more encouraging injury update.
Lewis limped off after 12 minutes during the 3-1 defeat at Liverpool and was replaced by Matt Ritchie.
It came at a bad time for the £15million left-back, who had started the previous four games under Howe.
“I don’t think it’ll be too long,” Howe said.
“It’s not a major injury but enough to keep him out of the festive period and some games into early January.
“Usually with a hamstring strain, you’re looking at minimum three to four weeks. That’s probably where he’ll be.”
Fernandez ended his six-match absence when he came off the bench early doors against Norwich City to help the 10 men of Newcastle claim a point.
He played the final five minutes when Newcastle clinched their first victory of the season against Burnley.
Following Jamaal Lascelles’ error at Leicester City, there were calls for Fernandez to be recalled to the starting XI, however a thigh injury before the Liverpool fixture has ruled him out.
Howe says Fernandez’s injury is “weeks rather than months” with a return hoped in late-January at the latest.
Howe said: “It’s a difficult one to put a timeframe on, but I think it’s more weeks rather than months, so we’ll wait and see how he responds.
“He’s definitely going to be out of the Christmas period, and probably most of January, I’d say, or the start of January, at least.
“He’s working hard and improving every day, so I think he’s in a good place mentally. He’s a very important player for us in every aspect because of his leadership qualities.
“The group love him and the supporters love him, so he’s a very important player.”
Howe was not asked specifically asked about Dummett ahead of the Man United clash, but training pictures in midweek showed him to be making good progress.
It is unclear when the Geordie left-back will return to the matchday squad, although all the signs are positives.