The Auckland franchise surrendered a 19-point half-time advantage to lose 29-24 to the Hurricanes at Westpac Stadium in its final Super Rugby match of the year on Saturday, which could prove to be the last outing for Williams and Nonu.
MacDonald's team was already out of play-off contention but capitulated against a team without key players including Beauden Barrett, Jordie Barrett and Ngani Laumape.
Former New Zealand centres Williams and Nonu will be out of contract after the Rugby World Cup in Japan and MacDonald is preparing for a significant loss of experience in the midfield.
"There are going to be some personnel changes, that's just natural, every year there are going to be some," he said after the loss to the Hurricanes.
"We've got our six or so [leaving], but now we've got a good understanding of where we feel we need to bolster, we can go and attack that area.
"We could be losing a lot of experience in the midfield, we're not certain of that yet, but we've got to get the right people in that midfield to help control the game with the 10s and have some good game understanding."