DALLAS – Luka Doncic fouled out of the Game 3 in the NBA Finals.

Here’s the problem: Doncic can’t foul out. Not in the Finals. Not with the game close late in the fourth quarter that Dallas ended up losing 106-99.

The Dallas Mavericks star is too valuable, too indispensable.

Doncic missed the final 4:12 of the fourth quarter after picking up his sixth and disqualifying foul with the Boston Celtics up 93-90. He picked up his fifth foul 26 seconds earlier and objected to the idea of getting six fouls. "You know, six fouls in the NBA Finals, basically I'm like, ‘C'mon, man. Better than that," he said.

But that’s irrelevant. The Mavs needed Doncic, who fouled out for just the third time in career, on the court, and he wasn’t. The Mavs were outscored 13-9 in those minutes without their All-NBA guard.

Criticism of Doncic, 25, is mounting with each Mavs loss in the Finals. They are down 3-0, and the Celtics can sweep the Mavs in Game 4 Friday (8:30 p.m. ET, ABC).

ESPN’s Brian Windhorst blasted Doncic, calling out his constant complaints about the officiating, his defensive shortcomings and an "unacceptable" performance in Game 3.

Now, Doncic is battling injuries. He was on the Game 3 injury report with a bruised chest and had been on the previous playoff injury reports with a sprained knee and ankle soreness. He’s a major reason the Mavs beat three 50-win teams in the Western Conference playoffs to reach the Finals.

He's All-NBA for a reason. His offensive talent is undeniable. But in his first Finals appearance, he’s learning what a different game it is – what’s required possession to possession on both ends of the court in the Finals.

Doncic’s stats in the Finals look OK: 29.7 points, 9.0 rebounds, 6.0 assists and 2.3 steals per game while shooting 47.3% from the field and 32.1% on 3-pointers. The Mavs need him to produce an inordinate amount of offensive production.

Drill down deeper and his numbers in the fourth quarter – two of the three games have been close – are unsatisfactory. He’s averaging 2.7 points and shooting 20% from the field (3-for-15) and 0% on 3-pointers (0-for-4).

He’s not the first star to struggle in a Finals debut. But it exposed how much improvement his game needs in these circumstances.

His frustration at officials and his defense are known issues. They're just more noticeable at this level. With all their offensive options, the Celtics are targeting Doncic. Boston's personnel creates advantages all over the court against Dallas, and Doncic is struggling to defend. Sometimes, the effort isn't even there as Jaylen Brown, Jayson Tatum and Jrue Holiday drive past him with little resistance.

"Yeah, he's definitely got a bull's-eye on his chest," Mavs coach Jason Kidd said. "He's got to be able to guard and understand that we're there to protect him and help him if he does get beat.

"Again, he's carrying a load offensively. They are putting him in every pick-and-roll and iso (isolations). He's got to be able to play the game where he can rest on offense and let others carry the load."

At this stage though, outside of Kyrie Irving, the Mavs don’t have the players to help Doncic offensively. At least against the Celtics.

"It's easy to point the finger at just him, say, you could be better. That's easy to say. I think he knows that," Irving said. "But, yeah, it's reiterating that I have his back. We all have his back."

The Celtics aren't strangers to playoff disappointments and the growth that comes from those losses. The hope is that Doncic realizes the changes he needs to make.