Sam Darnold is out Sunday with a quad injury, and his backup Drew Lock is nursing a sore elbow

The defending Super Bowl champions are down to a backup with a bad elbow. Sam Darnold will miss Sunday's game against the Arizona Cardinals with a quad injury, coach Mike Macdonald said Wednesday. He is week-to-week and won't go on injured reserve.

Darnold hurt the quad on the final drive of Monday night's 34-17 loss to the Bears. Macdonald also said Darnold cracked a rib on the second play of that game, when cornerback Kyler Gordon hit him on a blitz, and played the rest of the night anyway.

"This guy, he's tough as nails," Macdonald said. "He'll be back sooner than later, I'm sure."

The backup problem

The plan is for Drew Lock to start, but Lock has been limited in practice for weeks with an injury to his throwing elbow and was inactive as the emergency third quarterback the last two games. "He's able to throw. How much? We're working through that right now," Macdonald said.

Jalen Milroe will split practice reps and has to be ready if Lock can't go. Seattle also signed former UFL MVP Adrian Martinez to the practice squad Wednesday for depth.

The take

Losing Darnold might not hurt as much as it sounds. Since he came back from a glute injury in Week 3, he has thrown nine interceptions in six games, and his Total QBR of 50.8 ranks 22nd, per ESPN. He was 17 of 34 for 188 yards with two touchdowns and an interception against Chicago.

Lock, when healthy, has been the better passer this year. In the two games he played most of, he completed 72.9% of his passes for 422 yards, four touchdowns and no interceptions, including a Week 2 win at Arizona.

Seattle is 4-4 and has lost three of four. Macdonald said Darnold could be back for the Week 10 game against the Raiders, and the bye comes after that.