Exactly, it’s like people weren’t watching the Celtics. Center is not one of their concerns. Theis played well this year, and Kanter is fine as a backup big (if he opts in).
Ibaka for the MLE is the exact move I’d make especially if you could trade Aldridge for a big wing (and keep Poeltl). Alternatively, if you can find a S&T partner for Poeltl if you prefer to keep Aldridge. Ibaka should play mostly at the 5 and can stretch the floor, but excelled last year - albeit in limited minutes - playing alongside Gasol, so can spot you some minutes as a big lineup 4.