Ten days ago Michael Carter-Williams was the starting point guard for the Milwaukee Bucks.

Circumstances can change quickly in the NBA, as the 2013 Rookie of the Year found out when he was traded two weeks ago for the second time in his four-year NBA career. But Carter-Williams also realized just as quickly the value in carving out a role, and he made a great first impression on his coaches and teammates Thursday night in the Bulls’ 105-99 win over the Celtics.

The 6-foot-6 guard played a crucial role in the second half in slowing down a potent Celtics offense, and he provided an offensive jolt midway through the fourth quarter that helped push the Bulls to their first win of the season.

His numbers in 17 minutes won’t jump off the score sheet – he tallied five points, six rebounds and three assists – but his impact on the game was felt on both ends, and in his three stints he cemented himself as a key part of the Bulls’ rotation moving forward.

Carter-Williamas was thrown into unfamiliar territory late in the second quarter. Isaiah Canaan was the first guard off the bench for Fred Hoiberg, but after he missed all three of his 3-point attempts and the Celtics reeled off a 14-4 run to pull within three, 43-40, Hoiberg deployed Carter-Williams alongside starting point guard Rajon Rondo.

In the 10 days since Carter-Williams had joined the Bulls, his time in practice had been spent learning the point guard position. But with Rajon Rondo playing well in the first half, Hoiberg kept his starter in and substituted in Carter-Williams, a natural point guard, into a three-guard lineup with Rondo and Jimmy Butler.

Having Rondo next to him, Carter-Williams said, helped him ease into the Bulls’ offensive sets that wound up making a difference in the fourth quarter.

“He definitely helps me out a lot,” he said of Rondo. “He’s one of the smartest guys I’ve ever talked to about basketball, he knows everybody’s position, he’s a great point guard and I’m glad I get to learn from someone like him.”

Carter-Williams’ impact was felt immediately. The Celtics missed their next four shots, with their only points coming off free throws after Nikola Mirotic fouled Jae Crowder on a 3-point attempt. The Bulls pushed their second-quarter lead back to nine by the end of the half as Carter-Williams defended both Avery Bradley and Gerald Green.

That same substitution pattern followed late in the third quarter after the Celtics used a 14-5 run to pull within a possession of the Bulls. The Celtics took their first and only lead of the night on an Isaiah Thomas 3-pointer, but with Carter-Williams again playing next to Rondo the Bulls reeled off seven straight points to push the lead back to six, 75-69.

Carter-Williams opened the fourth quarter on the floor without Rondo, though Wade initiated most of the offense. Playing off the ball, Carter-Williams continued to press defensively while waiting for his opportunity to contribute on the other end.

And when he got his chance, he made the most of it. After missing his first four attempts, Carter-Williams was left alone on the left wing for a 3-pointer that he connected on. Butler then stole an Avery Bradley pass in transition and found Carter-Williams, whose floater in the lane pushed the Bulls’ lead back 12, 93-81, with less than 8 minutes to play.

[SHOP: Gear up, Bulls fans!]

When Thomas reeled off five quick points to pull the Celtics within five Hoiberg pulled Carter-Williams for Wade, who assisted on two consecutive Taj Gibson baskets before connecting on a 3-pointer with 26 seconds left to seal the Bulls’ home opener victory.

It was as successful a debut as a reserve point guard could have, with Carter-Williams logging the only positive rating (+3) among the five players who came off the bench for the Bulls. A 30-point first quarter allowed Hoiberg to play Carter-Williams alongside Rondo in the second and third quarters, and the results were evident, even with Carter-Williams playing a “new” position.

“I think it was tough for me because when I was first in there I was with Rondo, and so when he’s in there he runs the 1 and I play the 2,” he said after the game. “So a lot of times in practice I was learning the 1, and it’s pretty hard to learn everybody’s position in one week. So I was just looking for anybody to tell me where to go, what to do, and just try to make the best of it.”

Despite the unfamiliar spot on the floor, Carter-Williams did make the best of it. In nine minutes alongside Rondo, the Bulls’ defensive rating (points per 100 possessions allowed) was 99.1, an improvement from the 103.6 rating the team logged on the night. Carter-Williams’ individual defensive rating was 95.1, the second best number among guards behind Wade (89.8).

The numbers weren’t as solid on the offensive end, with Rondo and Carter-Williams together managing just 93.5 points per 100 possessions. But the Bulls’ surprisingly hot night from beyond the arc – 11-for-25 – allowed Hoiberg to focus more on the defensive end, where the Bulls wound up holding the Celtics to 99 points a night after they tallied 122 against the Nets.

And Carter-Williams still came up with two key passes late in the third quarter as the Celtics threatened. First he found Mirotic open for an 18-foot jumper on the left baseline. Then he grabbed a missed Wade layup and kicked it back out to Mirotic, who buried a 3-pointer in the final minute of the third quarter to push the Bulls’ lead to six heading into the fourth quarter.

“I love Michael’s game. He had a couple times where he had no idea what he was doing when I called the play, but that’s going to happen. He’s only been around the team a few days. But he played with a lot of poise,” Hoiberg said. “I like how he can get into the paint. He can get downhill. That’s something that every team wants.”

It’s an important year for Carter-Williams. In addition to him learning a new system on the fly, he’s in a contract year and said it’s a personal goal of his to cement himself as the backup behind Rondo.

With Jerian Grant inactive and Canaan sitting the final three quarters, Thursday night may have done just that. And whatever his playing time looks like going forward, or who he’s playing with, Carter-Williams is hoping to make the same impact he did Thursday night.

“I think I can really lead us no matter who’s on the court,” he said. “Whatever Fred thinks is best he’s going to do, and of course it’s a personal goal of mine to just be on the floor as much as I can.

“I was able to find my rhythm a little bit (in the second half) and just try to help the team. Defensively, whether it’s rebounding, getting steals; offensively, scoring or making the right pass or the right play.”