Martinez, acquired in January from River Plate, has two goals and four assists this season and has been subbed off in each of the past two matches.
“We are just halfway through the season,” de Boer said. “The first six months are always the hardest for everyone. He has a new coach. He has to adapt. It comes with ups and downs. We have a lot of confidence in his quality. He knows that he has to do better. He wants to do better. He’s working hard. We have a lot of trust in him.