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United States international DeAndre Yedlin suffered a knee injury in Newcastle's opening Premier League game of the season on Saturday.
Yedlin took an awkward fall while challenging for the ball late in the second half, and the right-back received lengthy treatment on the pitch before limping slowly off the pitch.
Television cameras then showed him putting minimal weight on his right leg as he walked down the tunnel with the help of a trainer, and the Newcastle Chronicle reported he left St James' Park on crutches.
Newcastle boss Rafa Benitez said Yedlin will have tests to determine the extent of the injury.
"It is still too soon to know for sure," Benitez said. "There is a problem with his knee, but we need to wait. The doctor has to assess him properly."
Newcastle had already used all three of their substitutes when Yedlin left the pitch and the 10 men remaining could not find a late equalizer in the 2-1 defeat.
If Yedlin, who started 31 league games for Newcastle last season, misses a significant amount of time, Benitez could turn to Jamie Sterry or Javier Manquillo to fill in at right-back.
Yedlin is the U.S. national team's first choice at his position, with 52 career appearances. He started a friendly in June against Ireland and came in as a substitute against France.