Kerfoot/Kappy/AJ plus Dermott is more than enough to get a decent top 4 RHD to pair with Rielly.
Or Dermott (with a full healthy off-season) may already be that guy (or have that potential) if they can't find a trade that makes sense.
Rielly played well with Hainsey. Leafs only need to find "Hainsey" but more mobile, and doesn't treat the puck like a hand grenade, in order to make a solid pairing.