Noah Dobson
Should the Leafs decide that they will go all-in at the trade deadline, and do it to acquire a defenseman, the first name that comes to mind is Noah Dobson from the New York Islanders. His size (6'4, 220 Lbs.) and skill make Dobson an ideal choice for the Leafs to add to their top defense pairing in the absence of Tanev.
Dobson is a right-shooting defenseman from Summerside, P.E.I, and was a first-round pick (12th overall) of the Islanders in 2018. He has six goals and 18 assists for 24 points in 48 games, but he had 70 points last year. The 25-year-old is averaging almost 24 minutes per game this season, and like Tanev, Dobson is known to block a fair number of shots (83). (All stats from hockey-reference.com)
He is a pending restricted free agent (RFA) after this season, so the Leafs could control his rights for many more years to come.
Re-signing him likely won't be cheap, and neither will be acquiring him from the Islanders. The Leafs will need to put a package together that includes their 2026 first-round pick, one of their top prospects (Easton Cowan or Fraser Minen), plus at least one player from the Leafs roster to help make the salary cap situation work. Even then, it might not be enough.
Dobson's cap hit is $4 million this season. To be able to acquire him with his full cap amount, the Leafs will need to trade either Max Domi ($3.75 million) or a combination of two players such as David Kampf ($2.4 million) plus Nick Robertson ($875,000). Asking the Islanders to retain some of Dobson's salary will only increase their asking price.
(Salary cap information from puckpedia.com)