Substitute Mikel Merino struck in the 91st minute in Arlington to send Spain into the last eight for the first time since they won their only World Cup in 2010.
"For me, Lamine has played one of the most important matches of his life," de la Fuente said. "Beyond whether he was brilliant or not, for me it has been one of the matches that will help him grow the most."
Lamine arrived at the World Cup managing a troublesome hamstring injury and started Spain's campaign on the bench as they were held to a goalless draw by Cape Verde.
He returned to the starting lineup after that and scored once in a 4-0 win over Saudi Arabia.
Against Portugal, the Barcelona forward had three attempts on goal, including two on target, helping Spain maintain the pressure before Merino's late winner.
De la Fuente suggested that the physical toll of trying to stop Lamine may have contributed to Portugal defender Nuno Mendes' injury in the 56th minute, when he was forced off and replaced by Nelson Semedo.
"Lamine did a spectacular job. He suffered for the team, he defended for the team. When he had the ball, he always created doubt and uncertainty for the opponent. He generated a lot of fear," de la Fuente said.
"We still need Lamine in this World Cup, to keep growing and delivering these kinds of performances," he added.
Spain next face the United States or Belgium.
Asked about a potential quarter-final opponent, de la Fuente said his focus remained on Spain's progress.
"Any opponent you face at this stage will have done enough to deserve being there, and whoever comes next will be an even greater challenge than those we have encountered up to now," he said.
FIFA World Cup 2026
The 2026 World Cup is taking place from June 11th to July 19th in the United States, Canada and Mexico. The tournament features 48 national teams and is played at 16 modern stadiums.
Match schedule and times | Group standings and knockout bracket | How to watch the World Cup | World Cup Format | Past winners of the tournament