Detroit Lions star running back Jahvid Best has been advised to sit out the rest of the season due to his second concussion in two months, according to a report.

Best suffered a concussion after getting tackled to the ground in the third quarter against the San Francisco 49ers last week. The second concussion in two months has caused doctors to advise Best against playing the rest of the season, according to CBS Sports.

CBS Sports reported that Best was getting advice that he needs to strongly consider sitting out the year.

The loss of Best for any substantial time period would be a major blow to the Detroit Lions after the team got off to a great 5-1 start to the season. The Lions -- likely realizing the desperation of the running back situation -- attempted to acquire the Eagles' Ronnie Brown at Tuesday's trade deadline, but the deal was voided because Jerome Harrison failed his physical.

ESPN's Adam Schefter reported on Thursday that Harrison failed his physical because of a brain tumor and is likely to miss the rest of the season.

 Without trade, Jerome Harrison doesn't get physical. Without physical, doctors don't find brain tumor. Trade might have saved his life, Schefter tweeted on Thursday.

The loss of the Lions two main running backs could mean that Maurice Morris gets the bulk share of the carries going forward. If Morris is unable to be productive, Lions coach Jim Schwartz realizes that it could put a damper on the team's highly-effective passing offense.

We had a hard time getting the run game going, Lions coach Jim Schwartz told the Detroit News after the team's loss to the 49ers. When it's one-dimensional that way (all pass), you are going to have a difficult time. And we did.

Best dealt with concussion issues during his college career at California, which might factor into the fears that those around him have about Best playing this season.

Best has rushed for 390 yards and two touchdowns this season. He struggled a bit in the team's first three games, but seemed to be rebounding with a Week 5 performance of 163 rushing yards against the Bears.