Per the Daily Mail's Sam Cunningham, FIFA are looking into "potential impropriety" after Real Madrid and Atletico Madrid were both handed transfer bans for breaching regulations over the signing of young players.
While "it is not believed Chelsea's case is as serious" as the two Spanish clubs', this marks the third time since 2009 that the Blues' dealings have been investigated by the governing body.
Chelsea were handed a two-window embargo in relation to the signing of Gael Kakuta in 2007, though their appeal to the Court of Arbitration for Sport was upheld, and Bertrand Traore's move attracted FIFA's attention when pictures surfaced last year of him turning out for the club prior to receiving international clearance at the age of 16.
According to Cunningham, the Blues have denied any wrongdoing, stating the club "complies with all FIFA Statutes and Regulations when recruiting players."
Chelsea can ill-afford a transfer ban if they're to keep pace with their Premier League rivals, all the more so as several first-team players will be in the final year of their contracts next summer—Thibaut Courtois, David Luiz, Pedro, Gary Cahill and Cesc Fabregas.
Should they leave and the Blues be unable to bring in replacements, it could be quite damaging to the club's prospects on the field.
That said, if it were to force the club to make use of the young players they so regularly recruit, there could be a silver lining.
As sports journalist Simon Austin noted, the Blues have a successful youth side, but it's difficult for managers of the first team to implement them into the senior squad:
Under a transfer ban, there would be more time to bring youngsters through in-house, though in the short term it could leave them struggling to compete for top honours.
One thing is clear—if Chelsea are found to have breached regulations again, the club will need to review the way they conduct transfers regarding young players to avoid future sanctions.
With regard to City, according to the BBC they have "no knowledge" of an investigation and are "adamant they have done nothing wrong."
As reported by the BBC, City are still awaiting a ruling from the Court of Arbitration for Sport over the signing of Benjamin Garre from Velez Sarsfield. The world governing body found City did not have a case to answer, but the case was taken to CAS and heard in July.
City have overhauled their youth system and have a number of talents coming through, but the path to the first team is tough with star names at every turn. Any transfer ban would offer potential to blood youth at the top level.