The Buffalo Bills have been impressive in the running game so far this season, the 513 total yards they've accumulated over four weeks good enough for sixth best in the NFL.
But the AFC East team's strong start in that facet of the game is in danger of being undone by growing injury concerns.
With star back LeSean McCoy already out with a hamstring injury, Karlos Williams - the man who has impressed in McCoy's absence - is now in danger of being sidelined.
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As confirmed by Buffalo General Manager Dough Whaley, via the team's beat writer Chris Brown, Williams is currently in the NFL's concussion protocol:
The news comes as something of a surprise as there didn't appear to be any concerns for the 22-year-old during Sunday's game against the New York Giants. However, Williams was diagnosed with the concussion on Monday evening.
Race against time
As with any player in the league, the running back must now pass all stages of the protocol before he is allowed to return to the gridiron.
Although this is possible to do in just a few days, there is a strong belief that Williams is now an extreme doubt to suit up for the Bills' week five game against the Tennessee Titans on Sunday.
The former Florida State star has looked good in his role as McCoy deputy so far this season. He has averaged 5.4 yards per carry on his way to three touchdowns on the ground, whilst adding another 46 yards and a touchdown through the air.
If Williams is indeed ruled out this weekend, the next man in line for the starting running back job for Buffalo would be veteran Anthony Dixon.
Bills fans: Can your team cope without two key men in the running game? Have your say in the comments below...