3rd ranked defense probably wouldn't have been possible even if healthy because the defensive Achilles heel is defensive rebounding, and none of the injury issues are really any that would have alleviated the defensive rebounding woes. 3rd ranked defense would also mean being a better defense than the SA Spurs, and that's just not happening this year, their personnel is superior, especially on the wings (Leonard/Green), and inside they are still better,
The difference between 7th (current BBall reference rank) and 3rd from basketball reference is 104.4 Drtg and 101.9 Drtg, the injuries don't make a 2.5 pts/100 difference in the defense. Their's the elite, Indiana, Chicago, SA, the just below elite, GSW, OKC, healthy Memphis (probably elite making a top 4 if Marc was healthy most of the year). Then I'd say the Clippers are knocking on the 2nd tier door along with some other teams, and are really good and but in a similar ball park to Charlotte, Toronto and Washington, and Miami who is 11th in Drtg is knocking on that door, but they have been a very inconsistent defensive team and aren't very good this year despite still being good.
They do have the potential to play much better defense in the playoffs though, that's the thing.
I know I'm giving Memphis more props despite having the same Drtg as the Clippers, but Marc Gasol their DPOY and defensive anchor missed 23 games, they're a different team on defense when he plays. Well, on everything.
While not to the level of Thibs and Chicago because he's too intense, but somewhat similar to what Pop does, Doc has helped the team to take injuries in stride and keep playing their game, and that has been great.