Coach challenges players to improve

Good job, Mike McGlinchey. Way to go, Fred Warner. Not too shabby, Marcell Harris.

And now that those kudos have been doled out, here’s another message for you: Don’t relax.

On Monday, head coach Kyle Shanahan sent the 49ers into their offseason with words that had particular resonance for their promising rookie class.

“I just tried to echo to the guys what is important and what I believe in: You don’t just show up four months from now and be like, ‘All right, it’s time to get better,’ ” Shanahan said. “It’s already too late. If you have that mind-set, you’re showing up to catch up.”

One issue in the 49ers’ justcompleted 4-12 season — disappointing years from players who had stellar rookie seasons — inspired Shanahan’s speech.

Most notably, cornerback Ahkello Witherspoon and free safety Adrian Colbert earned starting roles based on their 2017 debuts but regressed before landing on injured reserve. Backup quarterback C.J. Beathard was replaced by thirdstringer Nick Mullens after Beathard committed 10 turnovers in five starts.

Cornerback Richard Sherman noted many NFL rookies are eager to “exhale” after the end of a yearlong grind. It begins with training for the NFL combine, includes offseason practices and ends after their first season in the league.

“Sometimes you get lazy and you don’t have the same edge that you had the year before,” Sherman said. “I think the most important thing is to stay tight, and to keep your edge and to keep grinding. You were able to play this (rookie) year and be special. But it’s what have you done for me lately?”

Said defensive tackle DeForest Buckner: “Some guys go through their sophomore slump and it’s because they had such a long (rookie) year. And then they haven’t seen that much free time. Some guys … don’t really work out until OTAs come around. And they’re playing catchup when they get there.”

It’s folly to paint with a broad brush why the 2017 draft class, viewed as strong a year ago, is now regarded as shaky.

Wide receiver Trent Taylor, for example, had a dip in production after he underwent offseason back surgery. Inside linebacker Reuben Foster was released in November after his third arrest in a 10-month span. Much-scrutinized defensive tackle Solomon Thomas, a No. 3 overall pick, dealt with family tragedy when his sister committed suicide early last offseason.

Still, tight end George Kittle was the only member of the draft class to make a significant leap. After a 43-catch, 515-yard rookie season, Kittle had 88 catches and set an NFL record for tight ends with 1,377 yards receiving.

General manager John Lynch said Monday that Kittle laid the groundwork for his Pro Bowl season in his first NFL offseason.

“George Kittle started that the day he left here last year,” Lynch said. “He was on a mission.”

Kittle said his intense offseason was partly inspired by messages he received from Lynch and Shanahan. Both challenged Kittle to stay healthy after he was slowed by a variety of injuries as a rookie. Lynch and Shanahan even told reporters, unprompted, that Kittle’s durability was a concern.

“Yes, they did” challenge me, Kittle said. “They said your biggest goal is to see if you can stay healthy for an entire season. And that’s what I focused all my offseason towards. I’ll probably do the exact same thing” this year.

Shanahan has pointed to running back Matt Breida, who was undrafted, as another 2017 rookie whose strong offseason produced second-year dividends. Breida had 814 rushing yards and averaged 5.3 yards a carry, fourth in the NFL, despite playing most of his 14game season on a sprained ankle.

The 49ers hope this season’s rookies follow a similar path. It was a class headlined by McGlinchey, who was sturdy at right tackle, and Warner, who made the defensive calls and ranked 12th in the NFL in tackles (124). In addition, Harris was impressive at strong safety after spending the first eight games on injured reserve.

Harris’ late-season rookie push in the secondary was similar to that of Witherspoon and Colbert, who have relinquished their slam-dunk-starter status based on their second seasons.

“I want all of our guys to really focus on these four months,” Shanahan said. “How they are going to get better on their own. And we want to see it when they get back.

“If guys aren’t getting better, it’s not really a threat, but it’s reality: If you’re not getting better, it’s not going to be long for you.”

Eric Branch is a San Francisco Chronicle staff writer. Email: ebranch@sfchronicle.com Twitter: @Eric_Branch

Marcio Jose Sanchez / Associated Press 2017 Niners head coach Kyle Shanahan (left) is stressing that his players, particularly the rookies, need to keep their focus on getting better throughout the offseason.