Right-back: Juan Cuadrado (Juventus)
Juan Cuadrado has taken to his responsibilities as an attacking wing-back like a fish to water. He came out of nowhere as the answer to Juve's problems in that position since the departure of Joao Cancelo and has now made the role his own.
The 31-year old has provided four assists from that position, but it's the defensive contributions that have impressed the most as he averages 2.3 tackles, 1.1 interceptions and commits only 0.8 fouls.
He was on the brink of leaving Juventus in the summer but has now transformed into an indispensable member of its defence.
Center-back: Leonardo Bonucci (Juventus)
Juve's long-serving defender made a shock move to AC Milan in 2017, but returned to Turin just a year later and immediately regained his position. It was like he never left. And now he's having another fine season.
Bonucci is the only player from the squad to have started every league game - a clear indication of how important he is to Sarri's set-up.
He has also effectively stepped up in the absence of Giorgio Chiellini, who's out of action due to an injury, and has been a leader at the back. His return of three strikes is also a timely reminder of how deadly he is from set-pieces.
Center-back: Chris Smalling (AS Roma)
Chris Smalling became the latest English player to move away from the Premier League when he left Manchester United for AS Roma last summer in search of greener pastures. From his showings for the Giallorossi so far, it looks like he's buried all the frustrations of last season.
His aggressiveness and strong aerial ability have reinforced solidity in Roma's backline, as Smalling's overall gameplay looks better than what it has been in years thanks to consistent gametime. Roma might have already been convinced to turn this loan spell into a permanent one.
Left-back: Aleksandar Kolarov (AS Roma)
Signed for a paltry €5 million in 2017, Kolarov has made an absolute mockery of the fee as he's consistently been the league's best wing-back. He was the highest assist-making defender in his first season and then turned into the highest scoring defender last season.
With another five goals in this campaign, it's safe to say he's continued from where he left off last time around. Highly dynamic and productive, Kolarov shows no signs of slowing down even at 34 and was rewarded for his efforts with a contract extension last month.