The Spurs' interior defense must control Jonas Valanciunas and Jaren Jackson, Jr.
Two players that will be available for the Grizzlies that the Spurs have not had to face in a long time are Jaren Jackson, Jr. and Jonas Valanciunas. Valanciunas last played the Spurs in their season opener this past December while Jackson has not faced the Spurs since the NBA bubble back in August. Since then, he's been sitting with a torn meniscus and only just made his return in late April.
While Jakob Poeltl's development has been going particularly well lately, particularly with his free-throw shooting, he still has some trouble defending other centers that are bigger and/or significantly more athletic than he is, and I could envision Jakob having trouble with either one of Valanciunas or Jackson in the paint. That said, at this point, I am far more concerned with Valanciunas, who will assumingly be Jakob's primary matchup.
The Memphis Grizzlies are currently leading the league in second-chance points and Valanciunas is a big reason why, as he's currently averaging a career-high 12.5 rebounds per game and four offensive rebounds per game (third and second highest in the league, respectively). Given his sheer size standing at 7-0 and 265 pounds, Valanciunas is the kind of player that worries me when matched up with Jakob. His ability to play bully ball and impact both sides of the floor could severely diminish Jakob's production in more ways than one.
While I'm not nearly as concerned about Jaren Jackson Jr. on his own, what concerns me most about him is that he'll likely be playing alongside Valanciunas in the starting lineup. This could be a matchup nightmare for the Spurs as he has a significant size advantage on all of the team's normal starters that aren't named Jakob Poeltl. Even if Jakob is able to limit Valanciunas, Jackson's scoring ability could cause some serious issues on defense for the Spurs if someone isn't able to at least cause some disruption for him.