Expect the Celtics to make a trade, but it won't be for Russell Westbrook
The Celtics have the assets to make a midseason trade; It won't be for Russell Westbrook
The Celtics had two options when Kyrie Irving "communicated" he wouldn't be re-signing with Celtics. They could spread the cap space around and sign a few good to very good players including a big man, or they could use it all on an all-star to replace the one leaving. Danny Ainge wisely went the latter route. It's much easier to trade for good to very good players than it is for a star, so when you can add a Kemba Walker you prioritize that.
The Celtics quickly filled the big man void by signing Enes Kanter, Vincent Poirier, and re-signing Daniel Theis. Let's be honest here, the Celtics now have quantity down low, but not contender quality. All three of these guys can be your back-up big (using the term "big" here, because "power forward" doesn't really exist anymore, so your line-up on the court is typically a couple ball handlers, a couple of wings, and one big).
The main big on the Celtics roster to replace Al Horford come playoff time is most likely not on the current roster. Come the trade deadline when players signed this summer can be traded the Celtics will be making their move for a legit replacement for Big Al. Danny will have several young prospects and picks to barter with, plus a bunch of moveable contracts.
The Celtics are one trade away from being a contender. Right now at a minimum their five best players are all ball handler or wings. A trade is coming, but it won't be for the name being bandied about currently Russell Westbrook. I seriously doubt the Celtics would have wanted to replace one mercurial point guard in Kyrie Irving with another in Westbrook, and that was before they just signed a new all-star point guard. They can't trade Kemba right now anyway and they wouldn't even if they could. They're not going to pair Kemba and Westbrook in the backcourt and pay those two positions $85 million a year.
There actually is a market for Westbrook still; my guess is he'll end up on a team like Miami. When the Celtics make their next big trade it will be for a big. The Celtics needed a center and DeMarcus Cousins was available for a bargain basement price and Boston passed on him, because they didn't want the headache. They aren't going to add a very expensive headache at a position they are already set at. Westbrook will be traded sooner rather than later. He won't be coming to Boston.