I think they had to know going in. They've tried to fix the #2 centre slot over the past year and I don't think they gave up Byram if they hadn't planned for Mittelstadt's next contract. They're pretty clearly wanting a long term fix to the spot IMO. They tried hard to re-sign Kadri to a long term deal after the cup run but apparently EJ wouldn't waive his NTC to clear the necessary cap space.
Assuming Landeskog tries a return next year, the most likely guy they'll ship out is Lehkonen and potentially Manson. Folks over on the Avs' board have run the numbers a bunch of times (ongoing debate right now, actually). The Avs can afford 6.5 million or so for Mittelstadt provided at least one ~4.5 million dollar contract is shipped out and they add several cheap ~1 million dollar contracts to fill out other parts of the roster. If Landy comes back, then the Avs will have a ton invested in Rantanen/Landy/Nuke so I think a winger (Lehkonen, or Colton I suppose, but I think they'll keep him) will be traded. Manson and Wood are additional candidates who can be moved out.