This kind of move was a matter of time with the Leafs carrying nine defensemen on a full 23-man roster following the Jake Muzzin acquisition (12F, 9D, 2G).
Martin Marincin has shown reasonably well in spot duty while appearing in ten games and playing limited minutes (15:07/game) on the left side of the bottom pairing — arguably better than Justin Holl in his two appearances — but Holl is right-handed and the addition of another left-handed shot to the mix in Muzzin made this inevitable.
The other option was to send down Igor Ozhiganov, who is waiver exempt on an entry-level deal, but the contract he signed to come over from the KHL includes a Euro Assignment Clause and the team wants him working on his game in practice with the big club anyway (they’d be wise to rotate him in and give Hainsey the odd night off before the playoffs, but that would involve Babcock budging for a night as far as his reliance on him on the penalty kill).