West Ham were also keen on the player, however, City entered the race and moved fast to strike up a deal as they aim to add another body to their attacking line following the departure of Mahrez to Al Ahli last month.
City have already splashed out £107.6 million this summer, signing defender Josko Gvardiol and midfielder Mateo Kovacic. They’ve had to fork out another £55.5 million to make Doku their third signing of the window, but it's easy to see why last season's treble winners opted to do so.
Although still just 21, Doku has been carrying the reputation as one of Europe's most exciting young players for several years now. In fact, a move to the Premier League nearly came when he was still just a teenager playing for Anderlecht in his home country of Belgium.
He'd attracted interest from Liverpool in 2019, but rejected their initial advances on the basis such a move would come too soon in his career. It was a wise choice and he instead continued to play regular first-team football and hone in his impressive early potential.
The fine form he displayed at Anderlecht whilst still just a teenager - registering six goals and seven assists in just 37 matches - persuaded Rennes to break their club record transfer fee and part with £23 million plus add-ons to sign him in June 2020.
Two months later in September, he received his first call-up to the Belgium national side, which was no small feat for a player aged just 18. He scored his first national team goal later that month in a 5–1 win against Iceland; Doku has won a total of 16 caps since.
It's easy to see why City might see him as a natural Mahrez replacement. Although versatile enough to play on either flank, Doku is most often favoured on the right side of an attack. He's a direct runner who boasts scintillating dribbling ability whilst being able to deliver accurate passes and crosses into the penalty area.
In his first full season at Rennes, only seven players at the club played more Ligue 1 minutes, showing the faith he earned very early on at Roazhon Park under the stewardship of then-manager Julien Stephan.
He rewarded that faith with five goal contributions, whilst ranking top at the club in terms of progressive carries made, successful dribbles and second in the team for key passes completed.
That debut campaign looked to have laid the perfect foundations for him to kick on the following year, yet, a combination of injuries and managerial upheaval at Rennes - Bruno Genesio replaced Stephan in 2021 - meant Doku's game time was limited. He made just four league starts and accumulated a total of just 469 Ligue 1 minutes across all of the 2021/22 season.
Genesio was more restrictive with game time following Doku's return to full fitness in the early stages of the 2022/23 season and it took the youngster some time to build the same trust that he'd had with Stephan.
But in the latter stages of the campaign, Doku started to once again become a prominent figure. He signed off having registered five goals and two assists in his last eight Ligue 1 matches, and also earned a very impressive average Flashscore player rating of just under 8.0 during that streak.
In terms of a direct comparison with Mahrez, it's worth noting that Doku is actually right-footed, rather than left-footed like the Algerian international.
Yet, both enjoy hugging the touchline, rather than drifting into central half-space areas. This tends to be key in a team like City who enjoy a lot of possession and therefore need to try and stretch the opposition in order to open spaces through central locations for teammates to occupy and exploit.
Additionally, when he receives the ball in these wide positions, he can utilise his standout balance, skill and trickery to dribble away from opponents trying to dispossess him. An example of all this is below, taken from last weekend's 1-1 draw with Lens.
Note how high and wide Doku is, despite the position of the ball on the opposite side of the pitch.
The ball eventually works its way around to the winger who is then being closed down by nearby defenders quickly.
Doku's trickery kicks into gear and he dribbles away from the threat with ease before releasing a teammate unmarked inside the opposition's half.
Whilst there's much to admire about Doku's ability to get himself away from defenders and into dangerous positions, his efficiency in terms of his final ball delivery shouldn't be overlooked as it's the thing that really puts him at a level that makes him worthy of interest from the current European champions.
The fact he finished last season with the second-highest accumulated Expected Assist (xA) total at Rennes (4.9), despite 14 players playing more league minutes at the club, is a testament to this area of his game.
At Manchester City, he will be working with one of the finest coaches in football, as part of perhaps the current best-attacking unit in Europe, all of which is surely only going to enhance his current abilities and help him maximise his long-term potential.