The excuse becomes apparent when you consider he re-signed Zuccarello, the very proof-in-flesh that small players can be good top-6 players on contenders. He played in the Cup finals on the top line for the Rangers. He also didn't even crack an NHL lineup until 23 years old.
Rossi has simply been getting better every year, consistently, and doing all that is asked of him. Why would you not just wait until he's developed? Especially when you just re-signed a guy who fits the description of a small player who is not a top-6 player on a contender (anymore) entirely, and is in his late 30s. Like... you already have that, and you signed it yourself less than a year ago.
The Zuccarello re-signing tells me that Guerin isn't all about size and toughness, but it does show him to be a hypocrite when it comes to Rossi.