BOCA RATON, Fla. -- Detroit Lions coach Jim Caldwell admitted his emotions were many and varied when standout receiver Calvin Johnson told him he was going to retire.

"Think about it," Caldwell said at the NFC coaches breakfast at the NFL annual meetings. "You know what I mean? Think about it. Those are special moments because the great thing about football is you develop really good relationships. And it's not very often that you are involved with guys that mean so much to the game.

"Great competitor. But not only that, this guy is a world-class person."

Caldwell said the first inkling he had that Johnson might retire was shortly after the 2015 season ended. That's when Johnson mentioned he was pondering the possibility. The coach and receiver had several discussions about the pros and cons after, though Caldwell would not specify the exact number. Johnson announced his decision March 8.

Calvin Johnson is the Lions' all-time leader in receptions (731) and receiving yards (11,619). AP Photo/Rick Osentoski

"The most important thing is I think that he was certainly comfortable with his decision," Caldwell said. "He did the best thing for him and I think that's all that counts, to be honest with you."

Not many superstars retire at 31, but the receiver known as Megatron decided he had had enough. Johnson never gave a specific reason, but he has dealt with finger, ankle and knee injuries the last three seasons.

"He gave it serious thought," Caldwell said. "He weighed out all those options. He talked to the family members, agent, you name it. He did a thorough examination, but most importantly did a thorough examination of himself and made a decision I thought was best for him."

Caldwell reflected on a bit of Eastern philosophy he had just read while discussing Johnson.

"'Humility has to be the foundation of anything that you do if you want to involve greatness; the higher you climb the deeper your foundation of humility has to be,'" Caldwell said. "Calvin maybe was one of the most humble superstars that I've ever been around.

"Just a tremendous guy. So he'll be missed in more ways than just the fact that he's able to run by people, jump over the top of people and catch touchdown passes."

As for trying to replace Johnson in the Lions offense, Caldwell wasn't even going to say he'd try.

"There's only one Calvin Johnson," he said.

The Lions' major free-agent move, though, was to bring in former Bengals receiver Marvin Jones on a five-year, $40 million deal. Caldwell was quick to say Jones should not even think in terms of replacing Johnson.

"I think it's always something that you want try to make certain that you alleviate some pressure on a guy and let him understand exactly what you think," Caldwell said of Jones. "We do not anticipate that anybody is going to replace Calvin.

"I've been around a few great ones in my time, from Warren Sapp playing on the defensive line to watching Peyton [Manning] operate. There's not another Peyton Manning. Working with Calvin. Marvin Harrison.

"All those guys. You don't replace those guys. And you do have to let guys come in and be who they are, who they're capable of being and work in that order."

Caldwell enters his third season without a player he probably expected to be coaching for several more years.

"Our job is volatile. It's unpredictable. It's complex," he said. "And it's forever changing, and you have to be able to adapt."