Boston guard Derrick White, right, celebrates with Al Horford during the second half of Boston’s 120-108 win in Game 1 of the NBA Finals on Thursday night in San Francisco. Jed Jacobsohn/Associated Press

SAN FRANCISCO — The plan for Derrick White was always clear when the Celtics parted ways with draft assets to bring him to Boston at the trade deadline.

White’s shooting numbers were at career-lows with the Spurs, so there was some trepidation.

But the Celtics hoped the percentages would soar once he joined the team. The theory was that, playing alongside the likes of Jayson Tatum and Jaylen Brown, the open looks would lead to natural improvement. That wasn’t the case in the regular season, as White shot 31.2% on his 3-pointers. Even in the early parts of the playoffs, the 3-point misses mounted, though White continued to do all the little things to affect the game.

But White has transformed into a crucial piece for the Celtics in the last three games. He’s knocked down 3-pointers, continued to play solid defense and make plays. Most importantly, White continues to validate Celtics President Brad Stevens’ move at the trade deadline. The Celtics needed his 3-pointers Thursday in their 120-108 win over the Golden State Warriors in Game 1 of the NBA Finals.

“I missed my first two, but they felt great so I just stayed confident,” White said postgame. “Teammates and coaches have always told me to stay confident. It’s good to see that one go down, and just got on a roll from there.”

The Warriors dared White to shoot; he made them pay. Those makes add a different element to the Celtics’ offense, where either White knocks down the shots or he adds spacing for his teammates to operate. It just so happened White was on fire Thursday.

White was a difference-maker from the moment he stepped on the court. He finished with 21 points on 6-for-11 shooting, including hitting 5 of 8 from 3-point range. He added three assists and was a team-high plus-25 in his 32 minutes. It was exactly what the Celtics needed – especially as part of the impressive fourth-quarter comeback.

White has stepped up at the perfect time. He’s shot 11 for 18 on his 3-pointers over the last three games, finally punishing opposing teams for leaving him wide open beyond the arc.

The success was hardly guaranteed for White. He shot just 1 for 11 on 3s against the Nets in the first round as the frustration grew. But the makes are coming in bunches now, and the Celtics will take the points.

“Derrick is such a smart basketball player, he could fit in anywhere,” Tatum said. “I’m glad we got him. But he just knows how to play the right way on both ends of the floor. Just kept it going as soon as we got him.”

Of course, fans know all about White recently entering fatherhood. He missed Game 2 against the Heat because of the birth of his son, Hendrix, as he flew back to Boston early from Miami. White’s been a crucial sixth man for the Celtics ever since he took the time off – and fans have noticed the timing.

There’s some happy comedy and coincidence that comes from White becoming a dad. Even when he was traded from the Spurs to Celtics, his reputation was all about dad jokes. As the 3-pointers start to pile up, perhaps the dad jokes will, too.

“Derrick White is a baller,” Brown said. “He has been for a while, (going back to) San Antonio. He’s had a great series and playoff runs and things. He’s definitely a player that can elevate his level at any time.”