Midfielder Rafa scored the only goal of the game in the 63rd minute to upset Sam Allardyce's side in the Guadiana Tournament in the Algarve.
Following Monday's win against Sporting, West Ham could have won the three-team tournament but it was not to be for the Londoners on their second game in as many days.
West Ham could not find their best form and it possibly showed that they were resting regulars such as Jussi Jaaskelainen, Winston Reid, Matt Jarvis, Joe Cole and Mark Noble as substitutes.
And Portugal Under 21 international Rafa ultimately won the game shortly after the hour when reacting quickly when West Ham goalkeeper Adrian could only palm Yazalde's shot.
West Ham next face Portuguese side Pacos de Ferreira in a friendly on Saturday before the following weekend's Premier League season opener against Cardiff City at Upton Park.