Allen gained 700 yards in 10 games for the Washington Redskins last year, averaging 4.7 yards per carry.
But the 31-year-old runner has a history of knee problems of his own.
Picked by the Minnesota Vikings in the ninth round out of Clemson in 1990, Allen missed his entire rookie season after knee surgery. He also missed the 1993 season with a knee injury.
Edwards ran for 1,115 yards during his rookie season last year, but he severely injured his knee during a touch football game at the Pro Bowl in February. He will miss the entire 1999 season because of the injury, which doctors have said could end his career.
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