He suffered an MCL sprain in his right knee in the Patriots season finale against the Titans.
It's likely a 2-4 week injury. It's the same injury that has Jets cornerback Andre Dyson sidelined. Dyson's status for Sunday is unknown, but he's in Week Two of the injury, so this could help his chances of returning.
But even though Harrison will likely miss Sunday's game, expect Patriots coach Bill Belichick to list him as questionable. Belichick looks to mislead the media and most-importantly the opposing team with his injury report.
If Harrison is indeed out, the Patriots will either start James Sanders or Chad Scott at strong safety. But Scott is dealing with a back injury, and his status is unknown.
And making matters worse for New England is that they have already lost starting free safety Eugene Wilson to a season-ending groin injury.