The news comes in the wake of a transfer ban imposed on Chelsea earlier this month for a similar infringement of the rules.
Fifa revealed that Fiorentina has been in contact with them about Manchester United's signing of 16-year-old defender Michele Fornasier. According to the BBC though, the Italian club have made initial contact with football's governing body, no formal complaint has been made.
United are reported to be confident of their legal position and that Fornasier was not contracted to Fiorentina when he was signed earlier this year.
A FIFA spokesperson said: "We can confirm that Fiorentina have contacted us," said a Fifa spokesman. "However, so far no formal investigation was opened since the relevant documentation still needs to be completed. Only then will we be in a position to evaluate possible future steps to be taken."
