Wait, you can go on IR because of the flu? I don't think I've ever seen that before.
Is it like that crazy Swine Flu or Bird Flu or something? Or just an excuse to get Cross up on the big squad.
From what I've read (and I'd be happy to have others chime in if I'm totally off-base), it's at the team doc's discretion. In theory:
1) You can get pretty nasty complications from the flu, although that's usually with immunocompromised people (children, elderly, diabetes, etc.) For an ostensibly healthy adult like most NHL players, that shouldn't be an issue, but it can happen.
2) You could argue that you want to quarantine players ASAP to prevent it spreading. Having the sick player in Boston and the healthy players in Colorado is one of the best quarantine methods I can think of.
3) Maybe it's not the flu and it's something else. Maybe it's the flu in addition to something minor (say, tweaked elbow).
Either way, IR means he's ineligible for 7 days. It opens up a roster spot, and it may open up some cap space, too. He was going to miss Wednesday, and maybe they determined it's not worth it to send a half-healthy Morrow flying commercial across the country to Arizona on Saturday. He'll be eligible to play the game after that, which is Wednesday back at home.