Lammens faced United five years ago in an U18 friendly when with Club Brugge.
"Wow, I remember this," he told manutd.com. "I think it was the second team or the Under-18s, we played a friendly against them at home.
"It was a big day. The first team also played against the first team. Unluckily, I couldn't be there in the first team so it was really nice to play against the young guys. If you see some of the guys here, I see Tahith Chong and I played with him. You can see some big talents of that time. It was really nice to play against a team that, growing up, you looked at a lot.
"I almost forgot about it, but this picture reminds me of that time."
Lammens would eventually leave Brugge for Royal Antwerp.
He added, "To be honest, everything at Brugge was perfectly fine. I was really happy there and, from the youth onwards, I made big steps. But as a goalkeeper, sometimes you come into a situation where there is a big name in front of you and you have to be realistic, and maybe choose another path.
"You also have to have patience because, the first year at Antwerp, I didn't play, but, in the long term, I think it was a good option and looking back it was a good option, because the opportunities I got with Antwerp, and the learning process also, it got me to where I am now.
"I am very happy that I made the decision, but also, looking back on it, I have nothing bad to say about Brugge. Everywhere I went was positive."