PITTSFORD, N.Y. (AP) Buffalo Bills running back Boobie Dixon could miss up to a month after pulling his calf muscle, coach Rex Ryan has told The Associated Press.

Ryan provided the update Wednesday, when the Bills returned to training camp following a day off. Dixon was hurt Monday.

Ryan couldn't specify an exact timetable because he said calf injuries are ''usually tricky.''

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Dixon called the injury a ''minor setback'' in posting a note on his Twitter account Tuesday.

Dixon is a fifth-year player entering his second season with Buffalo. He was a dependable backup last year, when he had a career-best 432 yards rushing and a touchdown while filling in for injured starters Fred Jackson and C.J. Spiller.

This year, he's competing for a backup spot behind Jackson and newly acquired starter LeSean McCoy.

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AP NFL websites: http://www.pro32.ap.org and http://www.twitter.com/AP-NFL