According to Claudio Colla of TransferMarketWeb, who cites unnamed sources, the Argentinian's people have been in "long negotiations with both Fiorentina and Napoli" but are now in London to meet with the Blues, who are searching for a long-term replacement for John Terry.
Per the Daily Star's Alex Harris, the 20-year-old insisted he is not thinking about a move, but admitted he is open to a transfer elsewhere: "I am only focused on River Plate at the moment. Of course I’d love to play abroad but the only thing I am thinking about right now is playing for River."
Here are some of the up-and-coming star's best moments for River:
As the video reveals, Mammana possesses a reading of the game beyond his years and technical skill to match.
FourFourTwo's Jonas Giaever picked him out as one of 11 South American players who could make the step up to playing in Europe. He ran the rule over his strengths: "Classy on the ball, great technique and passing ability is backed up by strong awareness and the ability to be a step ahead of his opponent at all times."
Notably, he is just one of two players to have turned out for Argentina's national team before making his senior club debut—the other being Javier Mascherano. Further, Giaever believes he "could be the next great Argentinian defender."
He could still be something of a gamble, but with Terry set to stay for another year, the youngster would have the opportunity to develop his game alongside one of the best centre-backs the Premier League has seen.
Another option for Chelsea is Napoli defender Kalidou Koulibaly, and the Blues are ready to step up their pursuit of him after having a bid rejected.
According to Italian newspaper Gazzetta dello Sport (h/t Joe Short of the Express), Chelsea have had one unspecified offer turned down, but £23.3 million would be enough to tempt the Partenopei into selling.
Per Squawka, the centre-back won an outstanding 69 tackles and 81 aerial duels in Serie A this season, as well as making 82 interceptions and 125 clearances.
He has also shown great desire to get the ball forward, per WhoScored.com:
Football journalist David Amoyal believes the 24-year-old has come a long way:
Indeed, the Senegal international would likely be a more expensive capture than Mammana, though he is proven at a higher level and therefore less risky.
Even with Terry staying, it's past time the Blues secured a long-term replacement, and it's vital the club get the signing right.