The veteran forward signed a US$1-million, one-year deal with the Habs on Sunday after having the final year of his contract bought out by the Maple Leafs.
Armstrong's two seasons in Toronto were hampered by injuries.
The 29-year-old appeared in just 29 games last year, scoring one goal and three points.
However, Armstrong has a past history with Habs coach Michel Therrien and will be motivated after getting bought out.
He'll actually earn $2 million next season — half paid by Montreal, half paid by Toronto.