Italy coach Roberto Mancini was unhappy with supporters who jeered Leonardo Bonucci during the nation's 0-0 draw with Portugal in Milan on Saturday.
Bonucci, 31, spent last season at AC Milan after a move from Juventus, only to secure a return to Turin in August.
The move back to Juve has left a sour taste for some supporters, who targeted Bonucci during the Nations League clash.
Mancini criticised the fans and said there should never be a repeat when players are representing their nations.
"It was a mistake for fans to be booing Bonucci," he told a news conference.
"This is a type of protest that shouldn't happen when you're playing for the national team.
"Hopefully it only lasted for 15 minutes but I repeat it shouldn't happen because when Italy is playing, the crowd should support the players."
Italy will finish second in Group 3 of League A, with Portugal moving into the Nations League Finals.