MacLellan created the NHL salary cap space necessary to sign Carlson on Friday, when he cleared at least $7 million by trading goaltender Philipp Grubauer and defenseman Brooks Orpik to the Colorado Avalanche for the No. 47 pick in the draft.
Orpik has one year remaining on his contract, which carries a $5.5 million cap charge. Grubauer, a pending restricted free agent, received a qualifying offer from the Capitals for $1.5 million.
MacLellan said he thinks Grubauer could sign a contract worth about $3 million per season with the Avalanche.
"Both [Orpik] and [Grubauer] are really good people, well-liked by their teammates," MacLellan said. "You don't like trading away good people, but it's what we had to do to move forward with the team."
MacLellan said the Capitals now are about $21 million under the $79.5 million salary cap for next season. In addition to re-signing Carlson, they're hoping they have enough space to re-sign pending UFA defenseman Michal Kempny and restricted free agent forward Tom Wilson.
Forwards Devante Smith-Pelly and Travis Boyd, and defenseman Madison Bowey, are pending RFAs. Forwards Jay Beagle and Alex Chiasson, and defensemen Jakub Jerabek, are pending UFAs.
MacLellan said the Capitals also would consider bringing back Orpik if he is bought out by the Avalanche.
"We're trying to sign Kempny, we've got to sign Wilson as an RFA, and then make some decisions after that," MacLellan said.