As a fan, sure it is a concern. The win over the Jets would be the most emotional win of the year to this point, so an emotional letdown is a consideration. Add to that the short week, playing on the road, and in what looks like it will be bad weather conditions and the chances of a letdown would appear to increase.
I see a couple things that work in the Pats favor for a letdown not happening. First, they know the Bears are a good team - just look at their record - and therefore cannot be overlooked. Second, the Pats already had their dud letdown game against Cleveland - and that was recent enough they have not forgotten about it.
Overall I would say that a letdown is possible, but unlikely. Should be a good game Sunday.