The team is still waiting on results from Tannehill's MRI, a source informed of the situation told Rapoport.
Per the Miami Herald, Tannehill was scrambling to the sideline during team drills when he went down awkwardly. The signal-caller remained down for about 15 seconds before getting up and walking slowly to the locker room.
There are currently no further details on the injury.
"It looked like he sort of jolted that left knee as he landed on the ground. Then he crumpled into a heap right along the sidelines," Rapoport reported. "You can see the obvious and fast reaction from Ryan Tannehill -- throws the football away. The clear concern on his part, as you see right here him roll out and eventually fall to the ground. Clear concern on his part that this was an injury that he needed to worry about. No further word yet on what it actually is, but the vibe from the building from those who know Ryan Tannehill, at this point it is not good. They are clearly nervous, they are clearly worried about his injury -- that should be taken very seriously. And by the way, he was also wearing a brace on that left knee. He did have the partial ACL tear [from last season]."
"Just so everyone hears, the knee is really good. It feels really good," he said just last week, via the team's official website. "I haven't had any changes since the spring. I was a full participant in the spring and still going really well. Now we don't have to address that anymore, right?"
Matt Moore is still the Dolphins backup. The soon-to-be 33-year-old started four games for Miami in Tannehill's stead last season. While Moore remains a competent backup, if Tannehill misses any significant amount of time, it's a massive blow to the Dolphins' playoff chances. Adam Gase is a quarterback guru, but playing 16 games with a limited Moore is not what he had in mind entering Year 2.
After Jay Ajayi suffered a concussion earlier this week, a major injury to Tannehill would be a devastating blow to an intriguing offense.