(Reuters) - Buffalo Bills running back Fred Jackson has been ruled out of Thursday's game against the Miami Dolphins after being diagnosed with concussion, the National Football League team said on Monday.
Jackson was hurt after being struck in the back of the head with an elbow by linebacker Brandon Spikes late in Sunday's 37-31 loss to the New England Patriots.
"The tests this morning were not as good as we had hoped and he (Jackson) had hoped so he won't play this week," Bills head coach Chan Gailey told reporters. "It does make a difference.
"You have to make certain adjustments in a short week. The good thing is ... we have enough (backup) that we can go into a game with without Fred and have enough to be able to keep people off-balance."
In the absence of Jackson, Gailey said C.J. Spiller would handle most of the running back duties while Tashard Choice would also get "lots of opportunities".
"He (Choice) is a dynamic player to say the least," Gailey added. "He will get a lot of chances this week for sure."
The Bills have made a 3-6 start to the season while their AFC East rival Dolphins are 4-5.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Greg Stutchbury)