Brian Robb at BSJ looks at a couple of longshot possibilities that the C's could try to pull off if they are going to sign Walker.
Can the Celtics keep Horford and sign Walker? In theory, Horford's value to the Celtics goes up if they bring in Walker. Robb points out 2 ways that this could technically happen, though both of them are highly unlikely:
1. Trade Hayward into cap room. They would need to include assets to offset the possibility that Hayward doesn't bounce back. Robb calls this possible but highly unlikely, for numerous reasons.
2. 3-team sign and trade with Boston sending Kyrie out and getting Kemba back. Boston would need to ship out additional salaries to Brooklyn because Kyrie is a BYC player who would only count $20.1 million against the cap, plus they would need to send assets because why would Brooklyn want to help the Celtics unless the Celtics do something to make it worth Brooklyn's while.
Based on what would be involved here, Robb concludes (and I would agree with him) that it just isn't happening. He gives it 5% odds.
He also examines another option that seems somewhat more likely: Celtics execute a 3-team sign and trade that sends Horford out, Walker in. The advantage to doing this, from Boston's perspective, would be that it keeps them above the cap, meaning that instead of having the $4.7 million room exception, they would have the full MLE ($9.2 million) plus the biennial exception ($3.7 million) so they would be better able to compete in the market for available bigs.
This also seems unlikely, depending on where Horford is headed. If he goes somewhere where he can sign into cap space, then the Celtics would need to send assets to Horford's new team and to Charlotte in order to get both on board. That seems like it won't happen. But if Horford wants to go somewhere that is over the cap, then sign-and-trade is the only way that could happen, with Horford's acquiring team sending players to Charlotte to salary match. That team would be incentivized to make it worth it for Charlotte, so the Celtucs may not need to part with assets.so much in the way of additional assets