TAMPA – Carlton Davis is going to have to grow up fast. At 21, the Bucs need their rookie cornerback to come of age as a pro.In his first preseason game Thursday, Davis has a welcome-to-the-NFL moment. Facing second-and-16 in the first quarter, Dolphins quarterback Ryan Tannehill found receiver Albert Wilson in the right flat and he made a sliding catch, about 4 yards short of the first down.Sure, it wasn't the first time Wilson had taken a knee. (He kneeled for during the national anthem). But still believing it was college rules when a player is considered down where he hits the ground, Davis relaxed thinking the play was over. He stopped moving. He clapped once.That's when Wilson sprang to life, jumped up and ran for the first down.It's also when Davis realized the man to mannequin defense isn't in the Bucs playbook."That's the first time I've seen it and our whole sideline was screaming,'' coach Dirk Koetter said. "He was lucky he was on their sideline because he had about 200 people on our sideline screaming at him. When he came off, by the time I got to him, he said, "I know! I know!''"You know, that was one of those that no matter how many times it happens in practice, you just hope it doesn't happen again.''The Bucs found out Friday how much they are going to have to rely on Davis and fellow rookie cornerback M.J. Stewart.Vernon Hargreaves suffered a groin injury early in Thursday's game and is expected to miss 2-3 weeks. The Bucs believe the injury isn't serious enough to keep him out of the regular season opener at New Orleans Sept. 9."He's going to miss a little bit,'' Koetter said. "Soft tissue and those are always sort of delicate, tricky type. But it won't be anything I don't think that will keep him out from any real games.''It's a shame for Hargreaves, who missed seven games last season with a hamstring injury. He had been playing the best anyone had seen him perform since being drafted 12th overall by the Bucs in 2016.Veteran Brent Grimes, 35, also did not play Thursday. That means of the seven cornerbacks remaining, five are rookies and the others are Ryan Smith and Javien Elliott.The Dolphins attacked Smith after Hargreaves left the game Thursday. He had a pass interference penalty and gave up several completions. He did manage to produce the game-clinching interception on the final play."I think he struggled,'' Koetter said of Smith. "I think Ryan struggled. They definitely were throwing at him for a while on two of the drives they had. You know, the one (pass interference) they called against him down the sideline. I think you could reasonably argue that was (offensive pass interference) instead of (defensive pass interference) on the sideline. But you know, Ryan is working at it. Ryan is a tough guy. He had five tackles. He had the interception at the end of the game. But Ryan has got to be more consistent and no one knows that more than him.''Davis' mental lapse aside, the Bucs rookie defensive backs had a pretty decent NFL debut Thursday. Davis finished with five tackles and two passes defensed. Stewart was rarely out of position and had three tackles. Finally, safety Jordan Whitehead played fast and physical. He had two tackles and two passes defensed."I thought all three of those rookie DBs competed hard,'' Koetter said. "That's what you want to see. They all played physical, they all tackled well. They all have things to work on as well. We're pleased for their first game. I think the new players, the rookies in general, might have been a little bit nervous last night and that's to be expected. First NFL game. But those three DBs, they really did a good job.What did Koetter like the most about their play Thursday?"I thought the game wasn't too big for them,'' he said. "I thought they all played physical and I think all three of them had plays where the ball was in the air and left their feet and laid out to prevent a completion.''Now if they can only keep the growing pains from really hurting.