Its not how you start a season, it's how you finish it. And if the last 2 years are any indication, Giroux can really turn it on at the end of the season. He obliterated the Q in the playoffs and had a really nice series against the pens.
You forget that we've got Giroux, JVR and Briere in to replace them, right?
Couple things, though. Giroux and JVR, while projected to be good, have never been 20 goal NHL scorers. Knuble and Lupul were proven.
On top of that, Briere doesn't really "replace" Lupul and Knuble because Gagne's still a factor. Last year, Gagne came back and filled in for Briere (who supposedly made up for Gagne's absence the year before). Briere's pulling his weight and looks good, but we're essentially using Giroux, JVR, and Briere to make up for Knuble, Lupul, and Gagne. That's a lot to ask for right now.