I was an advocate for the Celtics using one of those empty roster spots to sign Chris Johnson, who is a tall and athletic if skinny big man with some people liked in his short stint with Boston.
Instead, he was signed to a ten-day contract by the Timberwolves, who were granted a temporary 16th roster spot due to a hardship exemption from having multiple players out for two or more weeks.
They have since signed him to a a second ten-day contract. Then, they will face a decision. I've read suggestions that they like him enough that they would consider trading the disappointing Louis Amundson so they can keep CJ (and Mickael Gelabale). Amundson is an energy big who rebounds and defends, but whose offense has gone into the tank this season. Some people were surprised he only ended up with the minimum salary.
[I do sometimes wonder what the season would be like if Rondo wasn't injured and the Celtics had used the MLE to retain Stiemsma and sign a big like Amundson instead of Terry while still being able to get Lee and Barbosa.]
Either has the potential to be an upgrade over Wilcox/Collins as the fourth big in the Celtics rotation. I'd prefer CJ because he has the potential to be a cheap option for the 4th/5th big with some upside and the team wouldn't have to give up a minimal asset to get him, if the T-Wolves decide not to sign him for the rest of the season. He's also the significantly better shot-blocker, but both guys have better shot-blocking stats than the Celtics' current best.
It's not a huge upgrade or a season savior, but it's an opportunity for a cheap upgrade on the current roster.