The 20-year-old joined the Giuseppe Meazza side from Vasco da Gama in July 2010, but initially struggled to make an impact in Serie A.
However, expectations have been piqued again after Coutinho impressed on loan at Espanyol in the second half of last season.
"Coutinho? He's a fantastic player. We all knew he was a bit special," Milito was quoted as saying on the official Inter website.
"The time in Spain has done him good and he'll help us a lot this year. He's different to other players."
Coutinho scored in Inter's 2-0 friendly win over Koper at the weekend.