RENTON, Wash. -- Seattle Seahawks quarterback Drew Lock has tested positive for COVID-19 and will miss Thursday's second preseason game, the team announced Tuesday.

The news came roughly 90 minutes after coach Pete Carroll declared that while Geno Smith was still leading Seattle's quarterback competition, Lock would start Thursday against the visiting Chicago Bears. Instead, Lock will have to isolate for at least five days in accordance with CDC guidelines, which the NFL is now following after putting its coronavirus protocols on pause.

Lock is vaccinated against COVID-19, according to ESPN's Jeff Legwold.

Speaking to reporters after practice -- before the announcement of Lock's positive test -- Carroll also revealed the Seahawks are hoping to have rookie running back Kenneth Walker III available for their season opener as the second-round pick deals with a hernia.

Lock's positive test was a surprising twist to what was already a noteworthy development in the battle to replace Russell Wilson. For the first time all offseason, Lock took all of Seattle's first-team reps on Tuesday in preparation to start Thursday. Lock, who had received only a handful of practice reps with the No. 1 offense until Tuesday, threw an interception during an 11-on-11 period and looked shaky in his most extended action with the starters.

Carroll said Tuesday that the expectation all along has been to start Lock in the second preseason game. Smith started the Seahawks' mock game at Lumen Field and their preseason opener at the Pittsburgh Steelers last weekend.

"This is part of the plan," Carroll said. "Regardless of what happened, we were going to give Drew a chance to start a game and just see what happens. Fortunately, he's played really well, and he deserves a shot to play just like everybody deserves a shot to play and show what he can do."

Carroll added of Lock: "He's been right on point. He's done everything that he needs to do to this point to position himself to be a starting football player. So we'll see how it goes."

Lock, acquired from the Denver Broncos in the Wilson trade, missed one game last season following a positive COVID-19 test and one in 2020 after he was deemed a close contact.

Lock outplayed Smith in the Seahawks' mock game and was having a strong night in Pittsburgh before losing a late fumble in Seattle's 32-25 defeat. He finished 11-of-15 for 102 yards and a pair of touchdown passes while playing the entire second half against Steelers backups.

"Drew came right out, took us right down the field and did well," Carroll said. "Popped the ball around and converted some third downs. He did good stuff. That's what's been happening. ... Pleased with both guys so far."

Carroll confirmed Tuesday that Lock was at fault for failing to account for the unblocked rusher who hit him from behind with a little over a minute left in the game, causing a fumble that Pittsburgh recovered. Carroll previously indicated that Smith was leading the competition because he was doing the better job of handling pre-snap responsibilities, such as changing plays and protections at the line of scrimmage.

"Yeah, he made a mistake," Carroll said of Lock. "He needed to read the guy coming off the edge, and he didn't do it right, so he got walloped. He'll never miss that hot again. That was a great lesson."

Smith went 10-of-15 for 101 yards and a rushing touchdown while mostly facing Pittsburgh backups. Carroll noted that Smith would have started 10-for-10 if not for a pair of drops and tight end Noah Fant failing to get his second foot in bounds on a well-placed throw to the sideline.

"Geno's still the No. 1 guy," Carroll said. "He's holding onto the spot at this point. I really like what Drew has shown us ... Both our guys can play. That's what we do know. They both can play."

Walker, who is expected to fill a significant role in Seattle's backfield this season alongside Rashaad Penny, was not seen anywhere on the field during Tuesday's practice.

"Ken's got a little hernia thing that he's working on, and we've got to get through that," Carroll said. "So I don't know what to tell you yet, but it's something that we can attend to and all that. We've just got to make sure that he's OK by the opener, is what we're shooting for."

Walker, the second running back selected in April's draft and the 41st overall pick, was having a healthy and impressive offseason to this point. Last Thursday, Carroll praised Walker for how well-rounded he has become as a running back. Specifically, he said Walker had "turned the page" on his pass-protection struggles from college, adding, "He could play all three downs and we'd feel comfortable with it."

Walker started against Pittsburgh, finishing with 30 yards on six touches. Penny didn't play against the Steelers due to groin tightness, but he practiced Tuesday.