The player has this week been linked with a move to Premier League side Arsenal.
Janssen has enjoyed a prominent rise in Dutch football since joining AZ Alkmaar at the beginning of this season.
He has managed 26 goals in 33 appearances for his side so far, with his performances having caught the attention of a number of top clubs in Europe.
Among them are reportedly Premier League side Arsenal, who the Mirror claims are scouting the forward ahead of a potential move in the summer transfer window.
According to McGuinness though, Celtic were far ahead of the competition in scouting him when he was still an unknown quantity.
“The summer window is crucial for the club both in terms of arrival and departure,” he told Sky Sports.
“There's one player, Vincent Janssen, who I was looking at. He was under the radar. He's now, a few years later, scored for Holland and is being linked with a lot of top teams.”
These comments are certain to annoy Celtic fans, who have had to endure a number of disappointing signings in recent years.
The striker department in particular has seen the worst of the recruits, as the likes of Nadir Ciftci, Colin Kazim-Richards and Carlton Cole have all arrived with little success.
Given the way Janssen has developed this season, he would have been a much smarter acquisition for the Hoops, who will regard him as an indictment of their poor transfer policy if he makes a big money move elsewhere this summer.