The Brazilian has been drafted into the position after John Stones joined Aymeric Laporte in the treatment room. Laporte is a long-term absentee, while Stones is out for at least a month, leaving Nicolas Otamendi as the only fit senior central defender.
Per Goal's Jonathan Smith, Fernandinho said:
"I'm still in an adaptation process. It's something new for me, although I did know since the end of last season that playing as a centre-back was a possibility.
"I've started training it from the beginning of this season so it's still new, but I'm trying to learn from that experience.
"The way we play makes it a bit easier because we keep the ball with us most minutes.
"But when an opponent attacks more through crosses or behind-the-line runs...let's see how I'm going to perform."
The 34-year-old has been deployed in defence on three occasions this season, which have ended in comfortable wins. He was in position for respective 4-0 and 8-0 victories against Brighton and Hove Albion and Watford, while he played in the 3-0 midweek triumph over Shakhtar Donetsk.
Former City defender Micah Richards and Sky Sports' Adam Bate were impressed with him against Shakhtar:
Football writer and broadcaster Ryan Bailey would have preferred manager Pep Guardiola to have called on one of his young prospects against the Hornets, though:
City are already five points behind Liverpool, so there's little margin for error.
As a holding midfielder, Fernandinho possesses attributes you'd want from a centre-back—strong defensive instincts and physicality, the ability to read the game and composure on the ball.
The Sky Blues have an away trip to Everton in their next Premier League game, but the Toffees are 14th. In their next five games in the English top flight, City don't face a side in the top half.
By the time they next face a top-six rival, it will be November and Stones will likely have returned.
Although it's not ideal to have a makeshift defender in place, Fernandinho is a useful solution in the meantime.
Following Joao Cancelo's arrival in the summer, Kyle Walker would also be free to cover there as City would still have an option at right-back. He has played as a centre-back for England before, albeit in a back three.
It is also an opportunity to give some minutes to their younger prospects, so that they have the experience to step in on such occasions in the future.