Webb was placed on the non-football injury list until he can pass the test. This comes following an offseason in which he missed organized team activities while his wife was in the later stages of pregnancy.
This continues a rough stretch for Webb. He missed most last year's training camp with a lower back and groin injury and struggled for most of 2014. Webb finished as the 78th-best cornerback, according to Pro Football Focus.
There was some uncertainty whether Webb would return to the Ravens. But he took a $2.75-million pay cut to secure his seventh season in Baltimore.
Webb's return has been delayed by the failed test, which measures anaerobic conditioning and the ability to recover.
"Webby is in really good shape in a lot of different ways," Harbaugh said. "I can see it in the way he moves in the weight room. But he's not in the kind of anaerobic shape he needs to be in to come out here in practice. The idea being that, if I go play after play after play and I begin to get fatigued and I can't move the way I need to move, then I'm going to have a fatigue-type of an injury."
Harbaugh said Webb took it hard and he's disappointed in himself.
"Yes, he's a little behind that way certainly and he's going to have to make up the ground," Harbaugh said.
Rashaan Melvin, who started four games last season (including two in the playoffs), replaced Webb with the first-team defense. Even though he struggled in the AFC divisional playoff loss in New England, Melvin proved to the Ravens that he can make an impact.
"I know one thing, Rashaan Melvin is a pit bull," Harbaugh said. "He's one focused guy."
Four players who were limited this offseason -- cornerback Jimmy Smith (foot), offensive tackle Rick Wagner (foot), center Jeremy Zuttah (hip) and wide receiver Michael Campanaro (quad) -- each practiced on the first day of camp.
“Very encouraged by all of the guys, including those guys [Wagner and Zuttah], that are out there,” Harbaugh said. “We’ve got to be smart with those guys as we go.”
After the news about Webb circulated in social media, former Ravens cornerback Fabian Washington tweeted that he knows from experience how tough the test is. It's a series of timed six 25-yard sprints.
You can be in great shape and fail the Ravens conditioning test. It's all about how you run it. If you go too ... http://t.co/cg7LSElMgY— FABIAN WASHINGTON (@FABEWASH31) July 30, 2015