COSTA MESA, Calif. -- Here’s how Los Angeles Chargers quarterback Philip Rivers summed up rookie receiver Mike Williams’ first year in the NFL.

“He never seemed fully, fully comfortable, and I don’t know that it ever just had flow to it, you know, for him,” Rivers said. “I still think there’s a little bit of thinking that’s going on. It never felt like he was playing free.”

Williams missed all of offseason work and training camp with a lower back disc herniation he suffered on the first day of rookie minicamp in May.

He returned to the field during the regular season and appeared in his first game action in Week 6 against the Oakland Raiders. However, as Rivers said, Williams never looked fully comfortable in the offense.

Williams also suffered a knee injury against the Dallas Cowboys on Thanksgiving Day that forced him to miss a game. The Clemson product’s disappointing rookie season wasn’t for a lack of effort.

“He worked at it now,” Rivers said. “But you can’t as a rookie receiver in this offense where there are a lot of things with multiple formations: ‘Now you’re the F, no actually you’re the X, oh Keenan [Allen] come back over here, play the Z,’ -- and we’re moving and we’re checking.

“It’s tough when you miss I don’t know, what, a third of training camp and practice and walkthroughs -- all those live reps, being in the huddle and all those things. We did the best we could of him being right there, kind of shadowing in the walkthroughs, and he was in meetings with Kellen [Clemens] and Nick [wide receivers coach Nick Sirianni] spent a lot of time with him. You just can’t really duplicate that.”

Williams finished his rookie season with 11 catches for 95 yards, including two drops. His lackluster effort was part of an uninspiring performance by the receivers taken in the first round of the 2017 draft.

Selected No. 5 overall by the Tennessee Titans, Corey Davis was slowed by a nagging hamstring injury, finishing with 34 receptions for 375 receiving yards and no touchdowns in 11 regular-season games.

John Ross, selected No. 9 overall by the Cincinnati Bengals, finished the season on the injured reserve with a shoulder injury and has yet to make his first NFL catch, playing in just three games his rookie season.

It’s not uncommon for receivers to struggle in their first season in the NFL due to the thick, complicated playbooks and complex coverage schemes they have to learn during their transition to professional football.

Selected No. 22 overall in the 2016 draft, TCU’s Josh Doctson played just two games his rookie season due to a lingering Achilles injury.

But in his second year Doctson was more productive, finishing with 35 receptions for 502 receiving yards and six touchdowns.

Selected in the second round of the 2010 draft out of Notre Dame by the Seattle Seahawks, Golden Tate was a healthy scratch his first five NFL games before finally picking up the offense enough to make his way onto the field.

A winner of the Fred Biletnikoff Award, given to the best college receiver in the nation, Tate finished with 21 receptions for 227 receiving yards his rookie season.

Tate eventually developed into a Pro Bowl-caliber receiver, with three, 1,000 receiving-yards season to his credit.

“Receiver is a tough position to come in this league and play right away,” Chargers GM Tom Telesco said. “It’s even harder when you miss OTAs and all of training camp, so we gradually got him in there. He did learn a good amount this year.

“It’s going to take this offseason of working with Philip, working with the quarterbacks. He knows the offense, but there’s a difference between knowing the offense in a book and actually applying it on the field, getting used to the timing and reps with the quarterback, and that all will come this offseason. So at least we got him back on the field. He got some work but we’re expecting more next year.”

Rivers echoed Telesco’s comments.

“This offseason will be huge for him,” Rivers said. “I’m excited about Mike. I think he’ll add a lot and bring a great impact to our offense. But this offseason will be huge for him, to get him healthy, all those OTAs, a full offseason program, weight room and running. Mike can add another dimension to our offense.”