The 18-year-old, who was born in Belgium, could be considered as a future England player due to being eligible under FIFA residency rules.
The debate over Janzuaj, who could also represent Albania or Croatia, was sparked after he scored both goals in United's 2-1 win over Sunderland last Saturday.
"If it was me and I wasn't born in England, I wouldn't want to play for England," Merson told Sky Sports News.
"One minute the FA are saying they need to get players through and all of a sudden this lad scores two goals against the bottom side, let's not forget that, and he's the future.
"We need to get more players in the Premier League, which is the top league in world football in my opinion."
Arsenal and England midfielder Jack Wilshere waded into the debate by saying that only English players should represent the Three Lions.