He's been injured.
If his injury is fixed then he would be worth a league minium contract, unless you have to expose a more valuable younger defenceman to waivers by doing so.
The Leafs could send down either or both of Mermis and Rifai to sign Klingberg and bring McCabe back off IR.
LD: Rielly, McCabe, Benoit, and Ekman-Larsson.
RD: Tanev, Myers, Timmins, and Klingberg.
Mix and match, on either side, in any order you like.
Signing Klingberg doesn't cost the Leafs anything but money. There is no opportunity cost, in case something better comes along later.
There's nothing else in the system.