Courtois was forced off in the 71st minute at SoFi Stadium in Los Angeles after suffering a muscle problem, with 24-year-old Lammens replacing him between the posts.
Belgium looked set to take the match into extra time before disaster struck two minutes from the end. Lammens was unable to keep out Pau Cubarsi's powerful long-range effort, allowing Mikel Merino to react first and fire home the winning goal in the 88th minute.
Despite the heartbreaking finish, Courtois was quick to defend his teammate.
"I gave him a big hug. He's a great goalkeeper. You only get stronger from this," Courtois said.
"Eventually you cannot say much more to him or give him much more advice. He is a strong guy, strong personality. I'm sure he'll be fine.
"You know, he will have some holidays and then regroup in Manchester and have a great season."
Courtois also revealed he had hoped to remain on the pitch after feeling pain in his right leg early in the second half following a clearance.
The Real Madrid goalkeeper admitted he wanted to continue but respected coach Rudi Garcia's decision to make the change.
"Obviously, I wanted to continue, but the coach wanted someone 100 per cent," Courtois said.
"I wanted to try to play maybe five or 10 minutes because I was feeling good. I was making saves, and I was not disturbed to make those saves.
"So that's a decision of the coach, and that's not a problem.
"Obviously you're not happy when you have to leave a quarter-final of a World Cup. But yeah, sometimes it is what it is. You can't change it."
Belgium defender Brandon Mechele also stood by Lammens, insisting the mistake should not be blamed on the goalkeeper alone.
"It's a learning moment for him," Mechele said.
"It was not the most easy ball. I think it bounces just in front of him. And we as defenders could have helped him by following up maybe more.
"So he needs to learn from this, and we as a group we have to be there for each other and help each other."
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