The Leafs already have 6 dmen signed for next year that are all serviceable dmen. I know everybody is ragging on Rielly and he needs to be better this year, but he's getting more flak then deserved which would've been overlooked if our forwards could score more than a goal or two per night against Florida.
Rielly had a 45pt pace with a large decrease in powerplay time, more goals than Marner or Matthews, and top even strength minutes. The only playoff games where he actually looked really bad were the same ones where the whole team looked terrible. He wasn't great, but he wasn't nearly the biggest problem with this team.
Don't spend more money on defense, focus all spending on putting together the best forward group possible.