DALLAS — Harrison Barnes is in a tough position with the Mavericks. His best offensive position is at the four, where he can take slower bigs off the dribble and punish smaller guards on switches.

Defensively, it’s not always ideal. While the NBA moves closer and closer to position-less basketball, occasionally Barnes still has to muck it up in the paint with a premier post scorer, like he did Tuesday night against LaMarcus Aldridge. And Saturday night against Kevin Love. And last month against Marc Gasol — twice. You get the idea.

A good amount of the time, Barnes holds his ground. He is strong as hell and uses his solid base to surprise post-scorers who are trying to easily back him down for cheap points. It usually works and Barnes has good numbers guarding the post considering the amount of times teams go at him every game. Tuesday though, was not one of those nights.

Barnes did what he could, but Aldridge got the best of him during the Spurs 97-91 win. Aldridge finished 12-of-21 from the floor with a game-high 32 points, including making six of his final seven shots as the Spurs pulled away in the fourth quarter.

“He’s gonna wear you out down there, it’s a very physical game,” Barnes said. “It’s hard to stay out of foul trouble guarding him. I thought we did the best job we could.”

Barnes forced a missed jumper on Aldridge’s first post-up but it was curtains after that. Aldridge burned Barnes on a quick spin move on his next attempt and proceed to torch the Mavericks for the rest of the night, despite Dallas sending help with double-teams.

Barnes entered the game holding opposing post-up shots to a respectable 41.7 percentage. That number went up on Tuesday.

Having Barnes guarding bigs isn’t just making it tougher for the Mavericks defense, it’s also zapping some of the juice from Barnes’ offense on the other end. So far, it hasn’t been a big problem. While Barnes guarded Love for most of the night in the loss to the Cavilers on Saturday, Barnes still had a good shooting night, scoring 22 on 9-of-14 shooting and hitting two three-pointers.

“It’s tough. I thought we’ve kind of avoided a little bit of that this season,” Barnes said after the Cavaliers game. “[Guarding Love], we couldn't really get around it. It’s one of those things you really just have to bear down — one of these guys is bigger than you, they’re gonna back you down, wear you down all game. Offensively when you get shots, try to sit down, get your legs into it and trust your technique.”

It’s something Barnes reminds himself almost daily. Against Aldridge, it finally felt like there was an effect. Barnes had his worst shooting night of the month (5-of-14) and missed both his three-pointers. Dallas did a good job to get him in favorable matchups against the slower Aldridge, but there just wasn’t much bounce from Barnes. Dirk Nowitzki mentioned after the game that playing the Spurs defense is tough because they switch almost everything and with the Spurs playing lots of interchangeable guards and forwards with long wingspans, it was hard for Barnes to find a size mismatch to give him his bread and butter post-ups.

“It takes a toll,” Barnes admitted after the Spurs game. “My shots were a little short in the first half. I thought second half we did a different job with different matchups to help get my legs underneath me. That’s just what the matchup is, it’s going to be a grind.”

Despite Barnes being a sort of jack-of-all-trades for the Mavs, it’s not translating. The Mavericks are still rebuilding and Barnes is doing things that weren’t asked of him before he came to Dallas. Sure, he guarded bigs in the past in the Golden State Warriors small lineups, most notably against Zach Randolph when the Warriors were down in their series against the Memphis Grizzlies in 2015. However he didn’t have to follow up that on the other end of the floor by having to shoulder almost all of the offensive burden. He just needed to spot-up around the greatest shooting duo of all time.

It’s not as easy with the Mavericks and everyone knows it. It’s part of the reason Mavs coach Rick Carlisle and the Mavs front office is so enamored with Barnes in the first place — he welcomes the struggle, in hopes of it making him a better player down the road. With Dennis Smith Jr. getting better every game (a season-high 27 points against the Spurs) that offensive burden slowly but surely is transferring over from Barnes to Smith. Reinforcements are coming, it’ll just take longer than everyone wants.

“It’s a very tough job,” Carlisle said after the loss to the Spurs. “I believe he’s built for those kinds of challenges. That’s just his DNA, his fabric. But it’s tough, there’s no question about it.”