I like Football Outsiders' metrics as something else to look at but a lot of that is projecting that the defense will keep this up. While it can be possible for them to maintain elite production, so much of that is predicated on staying healthy. Imagine for instance losing Kyle Fuller, Danny Trevethan or Khalil Mack to injury. That sort of loss would make a real dent in the defense. Another thing that makes it hard to project out like that is the fact the variance in quality of NFL opponents can put a serious dent into those numbers once we face elite offenses.
All true, however a couple things to consider.
1. What you said about injuries holds true for just about every team and in particular, those who have injuries to high impact players.
And to be honest, I really think our defense has even more potential than we've shown so far. Maybe not in terms of the raw eye popping stats, but just in solid play.
2. While the variance will change as the season goes along and FO numbers always change accordingly, we really don't face many of those "elite" offenses this year. The clear exception being the game against the Rams. But even the Patriots and Vikings offenses haven't looked particularly good so far and the Packers and Bucs offenses so far are better than either of them, and we've already faced them.
On the FO offensive side, we still face 6 games against teams below us, 2 against the Vikings who are just one spot above us, the Dolphins, Packers again, and Patriots, all of which are not in the top 10 at this point.
Some of them are bound to have good games and get things going ... in particular the Patriots and Vikings will likely pick it up some. But I don't think we've going to have to face many more offensively elite teams this year.
I guess we'll see how it all turns out.