Kieran Trippier has been ruled out of Newcastle's "upcoming fixtures" after fracturing a bone in his foot during a 1-0 win over Aston Villa at the weekend, the club said on Monday.
The England full-back has been impressive for Eddie Howe's side since joining the Magpies from Atletico Madrid in the January transfer window, scoring twice in a three-match winning run that has lifted them out of the relegation zone.
The 31-year-old, who struck from a free-kick in the first half at St James' Park on Sunday, was substituted shortly after half time.
"Trippier underwent an X-ray immediately after the game and scans have shown a fracture to the fifth metatarsal bone in his left foot," Newcastle said in a statement.
The club's recent good run has lifted them one place clear of the drop zone, four points ahead of 18th-placed Norwich.
Howe said after Sunday's win that Trippier, who also scored a free-kick in last week's 3-1 victory against Everton, was "stamped on" and could not continue.
The right-back, who has won 35 caps for England, joined Newcastle for around £12 million ($16 million) - the club's first signing under their new Saudi owners.
Forward Chris Wood, midfielder Bruno Guimaraes and defenders Dan Burn and Matt Targett all followed him to St James' Park.
Howe said Newcastle's return of 11 points from their past five league games showed they were in a "good place".
"We've worked incredibly hard," he said. "In the last three games I don't think there's been any secret to how we've won the games.
"I don't think they've been fluent footballing performances where we've played from back to front and all the things that we potentially want to do in the future but it's been very disciplined, hard-working, a lot of energy, a lot of team-work, team spirit, togetherness that have made the difference for us."