While it's impossible to predict injuries, the Pats want to give themselves the best chance to be healthy heading into the playoffs. They certainly do not want a repeat of 2015, when the league's best team was wrecked by injuries late in the year.
Per PUP rules...
The earliest Lewis can return to practice: Monday, October 17
The latest Lewis can begin practicing: Tuesday, November 22
The earliest Lewis can make his 2016 debut: Week 7 versus Pittsburgh
The latest Lewis can make his 2016 debut: Week 14 versus Baltimore
If Lewis returns to practice next week, it's a great sign for his midseason availability. He's eligible to return to practice on Oct 17.