The Chelsea striker admitted this week that he is beginning to feel increasingly relaxed with the rest of the squad following a difficult first three years as a Spain international.
Costa failed to find form in their poor 2014 World Cup campaign and was omitted from the squad for Euro 2016 after poor performances in the 2015-16 season.
However, the 28-year-old - who has scored 17 Premier League goals this season to fire Chelsea towards the title - has hit three in his last three appearances on the international stage, and Lopetegui has been delighted by what he has seen of late.
"He's calm and happy," Lopetegui told Radio Marca. "He has had a lot of consistency and form at Chelsea and we are all delighted.
"The players who are with us have shown incredible behaviour. The players are the medicine for the Spain team and they will help us achieve our goals."
Spain take on Israel on Friday in World Cup qualifying looking for a win that will keep them above Italy and at the top of Group G.
But with Israel just a point behind the top two in third place, Lopetegui is well aware of the threat they pose.
"The main focus is to get points against Israel. We need to convey the importance of this game," he said.
"They have good players and they are a competitive team. We know how difficult they are, we have to perform well.
"We absolutely respect Israel and I'm sure that they are also going to be respected by the city of Gijon."