If Simmons returns, I won't be concerned about the defense, even without a rim protector..they'll still have the best group of wing defenders in the NBA, along with an elite system..
The team overall is a concern, though..most concerned I've been about the Spurs since the 2011 embarrassment..the projected starting lineup is going to feature 3 black holes(Murray, Gay and Aldridge), which is terrifying..Kawhi is a willing passer, but his specialty is 1 on 1(where he's probably the best in the league)..Murray isn't as much of a concern, since Patty will be receiving more minutes, but implementing a career cancer coming off an Achilles is going to be a daunting task..
Then they will have to implement Parker coming off a major injury
Having Kawhi and Pop is a guarantee that they'll do well during the RS, though..the bench will still be one of the best in the NBA, as well..