Yes, not using the TPE was a big deal. I don't know all of the information, so I'm not 100% comfortable blaming Brad, but *if* he was authorized to spend another $17 million, and didn't do so, then he should obviously be blamed for that.
We can't act like our choices were Kornet / Vonleh, or nothing. Yes, there are the current free agent centers: Favors, Cousins, Whiteside, D12, Zeller, Aldridge. But, there are also other guys that were available that the Celtics had no reported interest in. Among free agents, none were world beaters, but I'd take all of the following over Kornet: Robin Lopez, Markieff Morris, T.J. Warren, Jalen Smith (would have been in place of Gallo), JaMychal Green, Taj Gibson, Gorgui Dieng, Mike Muscula, Bol Bol, Deandre Jordan, Montrezl Harrell, Otto Porter (again, in place of Gallo). Some of those guys would have helped, others wouldn't have. The only one we had reported interest in was Harrell.
Then, there were trade possibilities, as mentioned by nick and others. Christian Wood is second among centers in scoring. We passed. New York would have paid us to take on Noel. KO may have been available. Jock Landale looks good right now, and he was traded for a minimal amount of cash.
Right now, we're getting 7.2 points, 9.0 rebounds, 1.8 assists, 1.6 turnovers, 42.3% FG%, 6.1 fouls and terrible defense out of Vonleh / Kornet / Griffin in 35 combined minutes per game. (Adding stats like that with different numbers of games played isn't 100% valid, but it shows the sheer lack of production).