Johnson, who had the game-winning catch in the Titans' comeback win at Detroit in Week 2, had been somewhat phased out of the Tennessee offense in recent weeks. He was held without a catch in the past three games and wasn't even targeted in one of those contests.
Still, Johnson made an impact with the Titans, despite just nine catches for 85 yards and two touchdowns.
Johnson had picked up the nickname "Sensei," according to receiver Kendall Wright. That word is a Japanese term for an older mentor type of person.
"It was shocking, but we all miss him already. He was like a big brother in the group. We all called him 'Sensei,'" Wright said. "We all found out in the meeting this morning from Coach (Mike Mularkey), but since then, we called him on Face Time and it seems like he's good. He was a great player, and I think he still had something left in him. But as you get older, you know your body and you know what you want to do.
"I think it's something each individual has to decide for himself. You don't know what somebody is thinking. You don't know what they're going through and how they're feeling. He's well respected around the whole league, so I'm sure he'll be good."
Johnson's retirement even caught Mularkey somewhat off-guard. Mularkey said Johnson called him Sunday night to request a meeting on Monday morning, when he broke the news to him of his decision.
"He reached out to me last night, asked if he could come in this morning. That (retirement) was one of the thoughts that I had. I had a number of them," Mularkey said of the call. "The unfortunate part of it, it was, that's what he felt. He said he's been contemplating it for weeks and his heart was not in it. This is a tough business to be in when your heart's not in it."
--The Titans find themselves in second place in the AFC South at the halfway point of the season. At 4-4, they trail the Houston Texans by one game in the standings.
For a team that has not been in the postseason since 2008 and is coming off a two-year run where it went 5-27, this is heady stuff.
"I'm very excited. It feels good to have an opportunity to play in the playoffs. That's one of our goals, to give ourselves a chance to be in the playoffs," tight end Delanie Walker said. "I think it energizes it a lot to have a chance to be in the postseason."
--With Johnson's sudden retirement, practice squad receiver Tre McBride is an option to bring up to the 53-man roster. If effort alone is enough, McBride could be the guy.
"Tre is very hungry. Tre works about as hard as anybody on our team every day. He has to simulate the other team's leading receiver, and he gives us a great look," Mularkey said. "And then, even today, it was more of offense than the Chargers. He had a good practice today running our stuff. I'd have no problem if Tre is who we decide to bring up."