The Belgium international was introduced for the final quarter of the game and caused constant problems before adding to Matthew Lowton's first half opener.
Lambert said: I thought Christian was unplayable. He was an absolute threat.
"The great thing is he's only 21, only a kid and still learning.
"He has a really good game for the national team against Croatia in mid-week and, if you're holding your own in that company, you're doing alright.
"Everything is a culture shock for him at the minute but he was excellent.
"The goal will give him confidence. The crowd will go with him. You heard the crowd's reaction when he came on."