Strength of schedule is calculated from the opponents projected fantasy points allowed for the DEF position. The best rating is +16 which gives the player the advantage against the defense and indicates an easy opponent, the worst rating is -16 which gives the defense the advantage and indicates a tough opponent.
The loss of Navarro Bowman certainly affects them but they're still a tough unit and will be fine. Dallas has a multitude of offense weapons so this is a difficult matchup out of the gates. Still a must start nonetheless.
The 49ers lost safety Donte Whitner and cornerback Tarrell Brown but found more than suitable replacements in Antoine Bethea (Indianapolis) and Chris Cook (Minnesota). Year in and year out this is a punishing defensive unit that doesn't rack up huge special team points but will amass close to 40 sacks and a handful of defensive touchdowns while limiting yards and points allowed.