Except Sanders sucks in coverage and he doesn't play LB. Two of the biggest reasons why the Pats lost that game. The Pats still would have been eaten alive by Manning to Clark.
Besides, if he played for the Pats he might have been IRed in that game. Sanders only played four regular season games in 2006. The Colts kept his roster spot open for 12 games eventhough they knew he was going to be out for 3 months. The Pats would never do that even for an elite player. So if he was with the Pats, he would have been watching that game in street close in the press box. He could have made a huge difference there.
The Colts are different team. They are holding a roster spot for Manning right now even though the chances of him returning are remote. What they did for holding a roster spot most of the season for Sanders was stupid, but they just lucked out that it worked out for them.
So if you think that having Sanders on the Patriots payroll watching that game from home was the difference in that game, then you have a point. Whether he was with the Pats or Colts in that season, there would have been no way he would have played for the Pats that game because there is no way Belichick keeps a roster spot open for 3 months for a player. The most I have seen him hold a roster spot for a player before giving up was probably six weeks.