DENVER – Daniel Theis has quietly become one of the most effective role players in the NBA.

The 6-foot-9 German forward, whom the Celtics quietly signed to a low-cost, high-reward deal this past June, is adjusting to the NBA game quite miraculously. He has been good, at the very least, all season, but he has been great throughout January.

Monday’s game against Denver, during which the big man totaled 11 points, four assists, three rebounds, a steal and a blocked shot during only 21 minutes of action, was just the latest installment of a month-long stretch during which Theis has made an unmistakably positive impact on the Celtics.

“He’s a winner,” said teammate Al Horford. “He’s playing like that, and those are the kind of guys that you want to have on your group.”

Through 10 games this month, Theis has averaged 13.5 points, 11.1 rebounds, 3.4 assists, 1.6 blocks and 1.4 steals per 36 minutes. During those contests, he has shot at an efficient 57.8 percent rate from the field and 50.0 percent rate from long range. Those are outstanding all-around numbers for a role player.

Theis has brought so much to the Celtics every night, and he’s a big reason why they sit atop the East with a 36-15 record.

“Man, Theis is my guy. Not only what he does on the floor but off the floor,” Jaylen Brown, who hit the game-winner Monday night, said with a smile before rattling off a long list of what Theis brings to the table. “Good energy, good guy, always smiling, making jokes, great energy on the floor, what he does in the pick-and-roll in the seams, makes the right plays defensively, he battles, and he should probably get a little bit more credit for it.”

He’s not flying under the radar anymore. Not with the way he’s playing.

As great as Theis has been all month, he has been even better during Boston’s last seven games. He has caught a hot streak from long range, where he shot 53.8 percent during that stretch, and he made and attempted career highs from beyond the arc Monday night with his 3-for-6 effort.

Theis has also become a high-level passer at his position. He dished out at least two assists during six of his last seven games, culminating with a career-best four Monday night. He had only six such performances during his first 41 games in the NBA.

Theis averaged 5.2 assists per 36 minutes during Boston’s recent seven-game stretch. To put that into perspective, Horford ranks second in the league among centers with an average of 5.8 assists per 36 minutes. Theis is right on his heels.

“I don’t think he’s going to be making the Larry Bird flip pass anytime soon,” Brad Stevens joked, “but he makes the right extra pass, he looks for guys, he can roll and catch it in the seam and kick it out. So he knows what to do.”

The big surprise in all of this is how quickly the German big man has adjusted to the NBA. “People forget, because we’ve got JT (Jayson Tatum) as a rookie,” Brown reminded reporters, “but Theis is a rook, too.”

Horford has been impressed by how seamless Theis has integrated himself into the Celtics. He was asked what has surprised him about Theis thus far, and he replied, “I just think his ability to transition to the NBA. I know that he played in a good league over [in Europe], but it doesn’t compare to coming here and having an impact.”

Theis, now 25 years old, was playing overseas at age 18. He credits that experience for helping to prepare him for his first NBA season.

“Maybe it’s because I played seven years professionally, so I know my role,” he said Monday night. “I know my strengths, so it’s easy for me to fit into the system. I would say that makes it easy for me.”

The game has looked easy for Theis throughout January. He has become one of the most effective and efficient role players in the league, providing the Celtics with reliable play at both ends of the court on a nightly basis.

“One of the best things you can be called in this league is consistent,” said Stevens, “and he’s very consistent.”

Few reserve big men in the NBA have been as consistent as Theis has been throughout January. He has been everything the Celtics could have hoped for, and then some.