The tight competition on the Liverpool front lines forced Minamino to leave for another team, actually Minamino is a good player, fast and agile but unfortunately he failed to get a main place in the Liverpool squad, his experience playing at Salburz used to be a reason for Monaco to sign him, Minamino arrival I think will make Monaco a better contender next season in Ligue 1, now the deal has been done for 12 million euros.
The tight competition to become Liverpool's first line of choice seems to be a reasonable reason why Liverpool should release their players to Monaco. But I think Liverpool are also trying to make the team's finances more balanced after Nunez was bought for a high price. The sale of Mane and Minamino will make financial conditions more stable but perhaps Liverpool will still be eyeing other players in the market.
I don't know much about the player, so I don't know how he actually performed at Liverpool. Regardless, Minamino is probably expected to be a fine signing for Monaco. There isn't much speculation and rumors about it, but it's done deal.
Nunez purchase of a record-breaking price certainly made Liverpool finances a bit drained and the sale of Mane and Minamino could at least re-balance their balance sheets, from the statistics on Transfermarkt, Minamino is actually a good player and he has scored 10 goals in 22 games this season, but indeed the competition in liverpool is quite tight because the front line of liverpool contains many great players and it is difficult for him to penetrate the main squad, I think monaco is a good choice for him and seeing the monaco front line now of course he will get more guarantees to play much if later the performance can be consistent.
Without many rumors circulating now indeed Minamino has officially become the property of monaco and he is contracted for 4 seasons.