Last season's Super Bowl-winning quarterback Darnold sustained both injuries during Monday night's 34-17 loss to Chicago, and his recovery will be assessed week-to-week.
"He's tough as nails... He cracked his rib on the second play of the game, so playing the whole game without even making a name of it, and playing his tail off and competing," said Macdonald.
The quad injury occurred during the last drive of Monday's loss, added Macdonald.
"He'll be back sooner than later, I'm sure," said the coach, who said there are no plans to put Darnold on injured reserve.
Reigning NFL champions Seattle have endured a mixed start to this season, sitting 4-4 and trailing the San Francisco 49ers (6-1) in the NFC West. Darnold picked up a gluteal muscle injury early in Seattle's season-opening game in September.
The Seahawks went on to beat New England in that clash, and Arizona the following Sunday, under backup quarterback Drew Lock.
Lock is expected to start this Sunday's rematch against the Cardinals, but is having his reps limited in training as he recovers from an elbow injury.
Third-string QB Jalen Milroe is also being prepared for this weekend's game in case needed, said Macdonald.
"We'll be getting both guys ready to play. Got full confidence in those guys, and we have a plan in place, and we'll go execute," said the coach.
Suffering a second injury this season has been a "challenge" for Darnold, said Macdonald.
"But I just got a lot of respect for how he goes through these things - because he goes through it, you know, there's no blink, there's no flinch," he said.
"It's very matter-of-fact, attacking mentality, which I think is reflective of the rest of our team."
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