Los Angeles also signed promising goalie Patrik Bartosak to a three-year entry-level contract.
McNabb is expected to join the Kings' AHL affiliate in Manchester, but general manager Dean Lombardi sees a major role for the tough-nosed defenseman in Los Angeles in the near future.
McNabb has played in 37 games for Buffalo since late 2011, recording one goal and seven assists. He has played 12 games for the Sabres this season and 38 for their AHL affiliate in Rochester.
"Given where our team was at, it was going to be at some point critical to get a young defenseman who has a chance to play in your top four," Lombardi said. "He's closer to being ready than if I had to go after a kid who was still in junior."
Lombardi also said part of McNabb's game is being "overaggressive," but the GM likes that: "You'd rather tame a lion than paint stripes on a kitty cat."
Fasching was a fourth-round pick by the Kings last summer. The freshman forward at Minnesota has 12 goals and 15 assists in 31 games.
Deslauriers was a third-round pick in 2009 who moved from defenseman to left wing this season in Manchester for his first professional season. He has a team-leading 18 goals and 21 assists in 60 games for the Monarchs.
The Kings also acquired minor-league forward James Livingston from San Jose at the deadline for a seventh-round pick in 2016.