One thing that doesn't make sense to me about waiting for the buyout market to use the extra roster spot is that Olshey has said he wasn't going to make decisions based on just this season. Signing someone in the buyout market doesn't give you bird rights to that player so it usually leads to that player leaving the following season, like Kanter last year. Add to that the unknown of who will even be bought out and then which of those would even consider Portland in their current state.
This is why
@tester551 is probably on to something about the financial part of using the roster spot. It's also why a 2 for 1 trade like Covington and Dieng for Whiteside makes way too much sense because it adds an extra player without changing the cap number.