Lansbury stars for England U21 hours after leaving Arsenal
England's victory was inspired by midfielder Lansbury, who scored twice and set up both of Craig Dawson's two debut goals.
Pearce said of Lansbury, who was part of the squad in Denmark: "Henri, in some ways, came of age this summer.
"You have to look at these young players and sometimes it takes some longer than others to come make a stamp at under-21 level.
"Henri's one of those. He's been patient, his training was fantastic during the summer.
"We saw a different Henri Lansbury, a more mature young man that was playing and was fantastic around the group.
"We were expecting a great deal of him tonight and he's delivered."
Lansbury joined West Ham from Arsenal yesterday on a season-long loan and Pearce added: "The more players that we've got out actually playing for clubs, the better it is for me.
"For me as a national coach, it's ideal.
"As we've seen with the likes of (Theo) Walcott, the under-21s have had to supplement the lack of club football in some ways."
It's the home stretch! 24/7 click for the best coverage of Arsenal's final transfer market moves