According to Rick Sadowski of NHL.com Nathan Mackinnon will be sidelined for the next 7-14 days with a lower-body injury. (source)
There's an argument to be made that this is a worse blow for fantasy owners than it is for the Avalanche. Colorado has suffered a rash of injuries over the past month and has lost 216 man-games over the season. Losing Mackinnon for fantasy playoffs is a devastating loss and will be difficult to replace production. He won't be eligible for an IR spot either so he will remain have to remain in lineups which makes the situation even more difficult. Do what you have to do to stay in it.