COSTA MESA — The morning after his first day in the NFL, Mike Williams awoke to find that he couldn’t get out of bed.

Just a back spasm, he figured. The day before, his lower back had tightened up within the first hour of practice. The Chargers held him out as a precaution. But Williams, the seventh pick in last April’s draft, didn’t think much of it at the time. The excitement of his first NFL practice muffled any other emotions.

“I figured it was something that would come and go,” Williams said. A few days of rest, tops, he thought. “And that’s what the trainers thought, too.”

But after that first painful morning, the stiffness lingered for days, and then weeks. Every day, he woke to the muscles in his lower back aching, pain radiating down his legs and into his hamstrings. “I had to get a long stretch in before I could stand completely up,” Williams says. The team sent him to get an MRI.

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Meet this week’s Chargers opponent: Denver Broncos Six months later, Williams is standing outside of the Chargers locker room, happy to declare — finally — that he’s “full go.” The pain is gone. The mild herniated disk which held the rookie receiver out of OTA’s and training camp, through the preseason and more than a month into the regular season, is fully healed. He reiterates this, just to be clear.

It was a long road back, you see, from that first practice in May to his debut last Sunday in Oakland, and in those months the hulking 6-foot-4 rookie was relegated to the sidelines, hoping for the stiffness in his back to cease, Williams has heard the gamut of varying reports about his injury.

Back injuries can be fickle and vulnerable to setbacks, and so, any news was often ambiguous and open-ended, left up to interpretation. Some reports suggested he may not play this season. The Chargers assured him he shouldn’t rush his recovery, but Williams admits he felt the weight of the team’s investment in him.

“Being the first-round pick, you have high expectations,” Williams says. “Not being able to show what you’re capable of doing, just sitting back, watching, that’s tough. I’m a competitor. I wanted to be out there.”

Still, he knew he had to be patient. He would be ready when the time came, Williams told himself. But until then, he would have to rest. And wait. Lots and lots of waiting.

For most rookies, this might be a terrifying prospect, watching idly as your team careens ahead without you, but for Williams, there were no signs of panic. Growing up in the tiny town of Vance, South Carolina, with its two stoplights and population of a few hundred, patience came naturally.

Living now in Newport Beach, Williams doesn’t leave the house much. If he’s not watching film, he’s often playing video games. “I don’t really do much,” he admits. His demeanor is one of perpetual chill.

“Mike is so nonchalant,” said Chargers receiver Artavis Scott, who was Williams’ teammate at Clemson. “He doesn’t get caught up in a lot of stuff. He just minds his own business and takes it all day-by-day. He’s always been that way.”

That mettle met its first major test two years ago, in Clemson’s season opener against Wofford, as Williams made a toe-tapping touchdown catch in the back of the endzone and a defender shoved from behind, sending him headfirst into the goalpost. The collision fractured his neck and ended his season. As Williams was carted out of Memorial Stadium, with his neck immobilized and his body strapped to a backboard, Clemson coach Dabo Swinney worried he may never play again.

But even then, as Williams’ career hung in the balance, Scott was struck by his calm. For more than two months, Williams wore a neck brace, as the fracture healed on its own.

“Anyone else, they’d be freaking out, right?” Scott says. “He was just like, ‘Ah well, that happens.’ He knew there was nothing he could do except handle his business.”

A year later, Williams returned to Clemson, caught 11 touchdown passes, and the Tigers won the national championship. In the playoff final, Williams dominated an elite Alabama defense with eight catches for 94 yards and a touchdown.

It was that game-breaking potential that led the Chargers to draft Williams seventh overall — the highest pick on a receiver in franchise history — in spite of their impressive depth at the position. Some saw the pick as choosing a luxury over a need, and those concerns bubbled over as Williams sat and the Chargers lost their first four games.

Still, Williams remained unfazed. He knew his time was coming.

Then, last week, as the Chargers trailed the Raiders in the fourth quarter, Williams entered the huddle on third down. Rivers looked straight at him.

“Get there,” he said. “I’m coming to you.”

As the ball was snapped, Williams cut inside hard on a dig route 15 yards down the field. He was relaxed, calm, and as Rivers rocketed the pass, Williams reeled it in between two defenders. Five plays later, the Chargers scored to take the lead.

Still, after he received just 10 snaps in Oakland, Chargers coaches refused to get ahead of themselves.

“He did all right,” offensive coordinator Ken Whisenhunt said. “Once again, he’s a rookie. He didn’t have a lot of (reps early on). There are some things he’s got to learn about it. I’m, not saying that he did poorly. I’m just saying that he’s still (young).”

“It’s not like Mike has to come in and save the day,” Anthony Lynn added.

That shouldn’t change any time soon, as the Chargers receiving corps looks as deep — and healthy — as ever. The Chargers plan to continue taking it slow with their first-round pick. Against the Broncos vaunted secondary, Williams is expected to get around 20 snaps. From there, the team will take it week by week.

Williams is healthy. He’s ready. This weekend, when the Broncos visit StubHub center, seven members of his family will be in the stands, all the way from Vance, South Carolina. There are plenty of reasons why Mike Williams might be anxious for his role in the Chargers offense to grow — and fast.

But true to form, the eternally chill first-rounder from small-town South Carolina is willing to take the slow and steady approach, if need be.

“I feel like (Coach Lynn) is doing the right thing to not just throw me in there and say, ‘You’re starting.’ and play me the whole game,” he says. “These guys put the work in, and they’ve been in those situations before. He’s kind of just slowly working me in.

“But,” Williams clarifies again, “I’m ready.”