Spain coach Vicente del Bosque has called for "unity" before Sunday's Euro 2016 qualifier in Belarus after defender Gerard Pique was given a hostile reception by some La Roja fans in Leon this week.
Pique, who plays for treble-winning Catalan club Barcelona, angered Spain supporters who are also fans of Real Madrid by mocking Real forward Cristiano Ronaldo after Barca won the Champions League final last weekend.
Pique was whistled before and during Thursday's 2-1 friendly win over Costa Rica, prompting Del Bosque to leap to his defence and scold local media for what he said was fuelling "violence and hatred" between Spanish regions.
"I don't think what happened is right," Del Bosque told a news conference.
"Whistling one player is the same as whistling the whole team," he added.
"It leaves me with a very bad taste. The most important thing is that we are united."
Pique's treatment has its origins in the historic tension between Madrid, which represents central government, and Barcelona, where many are strongly in favour of independence from Spain.
Along with many Barca players who are also proud Catalans, like Xavi, Carles Puyol, Cesc Fabregas and Sergio Busquets, Pique has been a key part of Spain's recent success, helping them to a debut World Cup triumph in South Africa in 2010 and a second straight European Championship two years later.
They will be bidding for a third consecutive continental title at the tournament in France next year.