Lucas Torreira has made himself very popular at Fiorentina, and talks over a permanent transfer are set to begin imminently, according to a report.

Lucas Torreira moved on loan from Arsenal to Fiorentina this summer, with a reported €15m option to buy in his contract.
Things are going really well, according to Corriere Fiorentino (via Sport Witness), and it’s reached the point where Fiorentina rely on the midfielder’s involvement to play at their best.
Torreira has convinced everyone at the Serie A club that it’s worth signing him on a permanent deal, and talks are reportedly set to start soon.
Exactly what these talks will involve isn’t clear. As mentioned, Torreira already has an option to buy in his contract, so Fiorentina could simply activate it at the end of the season to get their man.

It’s possible they’re planning to try and lower the fee, as previous reports have raised that possibility. But the ‘talks’ in question may also refer to the conversations with Torreira needed to finalise personal terms.
Even if Fiorentina settle for simply paying their €15m purchase clause, that doesn’t mean Torreira has to accept the move. He’ll still be able to negotiate and potentially turn down a contract offer.
So Fiorentina don’t need to convince Arsenal to sell (unless they want a lower fee), but they do need to convince Torreira to stay.