Following the United States' dismal failure to qualify for the 2018 World Cup, interim boss Dave Sarachan fielded a youthful line-up for the friendly in Leiria.
Things started well: USA's 19-year-old midfielder Weston McKennie – making his senior international debut – put the visitors ahead in the 21st minute.
But their advantage lasted just 10 minutes as Horvath let Portugal back into the game with a bizarre blunder.
The Club Brugge goalkeeper attempted to claim Vitorino Antunes's hopeful cross into the box, but instead the ball squirmed through his legs and into the bottom corner.
Horvath was subbed out for Bill Hamid by boss Sarachan at half-time to mark a disappointing night for the 22-year-old.
The friendly finished 1-1.
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