"Today he cleared just about everything that needs to be done. They have one more test that they have to do this afternoon. They feel like he'll be cleared by the end of today," Frazier said after Wednesday's practice.
Cook suffered a concussion on third drive of last Friday's game against the and didn't return.
Veteran Chris Carr and rookie Josh Robinson will receive elevated reps in Cook's absence. Frazier said Carr is likely to get the start, but both of them will play in the nickel defense, along with starter Antoine Winfield.
Carr is entering his eighth NFL season and is the second-most experienced defensive back the Vikings have, next to Winfield.
"He's a very bright guy when it comes to understanding football and the concepts. His experience helps him a lot. Some of the things we do are a little bit different from where he's been, but he's adapted very well and it's good to pick his brain on some things as well," Frazier said. "He's a sharp football player."
Carr, who has spent time with Oakland, Tennessee and Baltimore in his previous seven NFL seasons, is playing more zone than he did previously but offers the team the ability to play more man defense as a change of pace.
NO STRUCTURAL DAMAGE FOR GUION
Letroy Guion, who entered the offseason as the Vikings' starting nose tackle, aggravated a sprained posterior cruciate ligament at Tuesday's practice and will be held out of the third preseason game Friday night, Frazier said.
"It flared up again so we just need to give it a little more time before we put him back out there again. He'll get this week in (for practice) and a few days next week as well," Frazier said.
Frazier said Guion had an MRI and there was no additional structural damage to the right knee. He is expected to be ready by the regular-season opener.
SHERELS IN THE LEAD
Punt return is one of the remaining unsettled positions, but Frazier, like special teams coordinator Mike Priefer on Tuesday, indicated that Marcus Sherels is the leading candidate once again.
Sherels was the only Viking to field a punt last year, returning 33 of them for an 8.4-yard average.
"Right now Marcus would probably be our opening-day guy if we were at that point. But we've still got guys fighting for that position," Frazier said. "… We're still evaluating it, but Marcus is still ahead of the rest of the guys right now."
Josh Robinson, Jarius Wright, Bryan Walters and Stephen Burton could all get looks there, but no one has looked as sure-handed catching punts as Sherels.
Wright has appeared to struggle the most with fielding punts outside and fumbled another one during a short special teams session Wednesday, but Frazier praised the progress he has made.
"Jarius has improved dramatically from when we had him back in the early spring after he was drafted as a punt returner. His hands have gotten better," Frazier said. ""His concentration has gotten a little bit better and we've gotten him out there in some ballgames in these two games we've had. He hasn't had much production, but we're going to give him some opportunities in this ballgame."
Jerome Simpson was also fielding punts Wednesday, but Frazier shot down that possibility after practice.
"No, no, no. That's something he wanted to do. I guess he's done it in the past. But no, we're not looking at him being our punt returner or kickoff returner," Frazier said.
Tim Yotter is the publisher of Viking Update. Follow Viking Update on Twitter and discuss this topic on our message boards. To become a subscriber to the Viking Update web site or magazine, click here.