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Serbia are being overestimated here. They have had over 40 players go through the team in the past three years. The team they have now is without some players with more potential, who aren't there for difference reasons. They lack continuity and results, compared to Portugal. Since the beginning of 2017, Serbia have three wins in 11 games (both preparation games and qualifiers), only one by 5 or more goals (Austria 26-24, the Czech Republic 31-21 in qualifiers and 17-15 against Montenegro). They also have 8 defeats (Sweden 17-28, Croatia 17-21 and 12-27, Montenegro 25-22, 23-24, France 26-21, Spain 34-18, Germany 26-20). Portugal played three preparation games in March against the same rivals Serbia played in EC group stage (Germany 24-29, France 13-23, (8-9 at HT), and Spain 24-26). They got good results against Germany and Spain. Here in group stage against Montenegro, Portugal were just two goals behind with some 15 minutes left on the clock. In EC qualifiers, Spain played 25-25 with Switzerland and Switzerland trashed Serbia 29-17 in the 2015 U17 EC. Portugal played U17 EC that year and beat the Czechs 27-22.
Serbia have slightly better offensive numbers here but their defense is weaker. They had more technical faults and some serious declines. I see a narrow win for either team and I definitively cannot imagine Serbia winning convincingly.