It's simple, the Leafs need a true top pairing defenseman. It's been the story all season, and come deadline time, Brad Treliving needs to pull the trigger on a d-man.
Jakob Chychrun is a good fit, specifically due to his cheap contract and puck-moving skills, but there's no guarantee that he'll be the guy, especially due to him being a left-handed player
Chris Tanev has also been a popular name, as he is known for his shutdown abilities and also being a right-handed defenseman. The only downside is he is 34, and the Leafs might want someone younger. He is also more of a depth addition than an "needle moving" type of impact trade.
Some other popular names include Noah Hanifin (who has a similar skill set to Chychrun but it also a left shot) and Sean Walker (who is fine but may not be the true high-end defenseman the Leafs want).
Regardless, the Leafs need to go all in on a defenseman (or two) if they want any chance of winning the Stanley Cup this year.