The Blues will be without All Blacks lock Patrick Tuipulotu for the rest of the Super Rugby Pacific season, after a broken arm in Friday's win over the Highlanders.
Tuipulotu, 30, played the entire 80 minutes at Eden Park on Friday, and scored a crucial try in his side's 16-9 victory.
However, the former skipper will now miss the playoffs through the injury, starting with the Blues' clash against NSW Waratahs on Friday.
"He broke his arm, so he'll be [out for] six weeks," said MacDonald. "I thought he was really starting to hit his straps, both in a leadership role and a key player in what he brings to our team.
"He's a huge loss. I was speaking to him straight after the game, chatting away to him for 10 minutes, [he] didn't say a thing.
"Twenty minutes later in the sheds he was walking round with a cast on his arm. He's not a man of a lot of words. He doesn't give a lot away, he didn't say he had a sore arm.
"But he obviously finished the game with it broken and didn't squeak at all afterwards.
"He's a tough old rooster, but the boys were definitely disappointed for him."
In better news, though, Beauden Barrett will likely return after missing the last two games of the regular season.
The 32-year-old suffered a cut to his heel away to the Reds in Brisbane, before sitting out of the Blues' win over the Hurricanes on All Blacks rest.
MacDonald says that at a push Barrett could have played against the Highlanders, but was held back to be given time to heal.
"He looked sharp actually, he looked really good," MacDonald said. "If we really wanted to push him to the limit, we could have potentially put him out last week.
"The extra few days to make sure the wound had healed properly and didn't open up was all he needed to get him through.
"He trained with no restrictions today."
The Blues will name their side to face the Waratahs on Wednesday.