The French midfielder is set to come in as a like-for-like replacement for Amrabat in central midfield.
It has been reported that Man United chiefs are in Italy in order to finalise a deal for the Moroccan midfielder which is expected to go ahead today.
Transfer expert Fabrizio Romano claims that Fiorentina have reached an agreement with Sassuolo to sign the 25-year-old midfielder.
The signing of Lopez should see Fiorentina give the green light for the Amrabat deal to go ahead, as the two clubs have agreed terms this afternoon.
Lopez is similarly combative to Amrabat, but will serve a suspension upon his arrival in Florence after his red card against Napoli at the weekend.
Amrabat will still technically be a Fiorentina player, as United have only agreed a loan deal for the season, but there is an option-to-buy in the deal.
Fiorentina boss Vincenzo Italiano has left Amrabat out of the squad so far this season due to the ongoing speculation, which now appears to have been resolved.