The Belgium striker has been in impressive form this season, scoring 15 goals for the Villa Park outfit since joining from Genk last summer.
However, Lambert has revealed that he was not intending to watch the 22-year-old when he travelled to Genk for a scouting mission.
"We signed him because I thought we needed a striker like him at the club," Metro quotes Lambert as saying. "I felt we needed a targetman so we looked around and saw Christian playing at Genk.
"He actually played with a strike partner that we looked at beforehand. We were looking across several different leagues at people but went back to watch Christian again and things moved from there.
"We watched him more and one of the scouts saw him playing for Belgium and we decided to take a chance."
Villa currently sit 18th in the Premier League table.