If Tua was your normal, run-of-the-mill draft pick, there would be no problem with letting him sit all year. BUT, Tua suffered a serious injury and has not played a snap of football since he was hurt. The doctors have cleared him, so they must think that he's healthy. However, NO ONE can 100% guarantee that he can take a big hit and keep playing; it's possible that a big hit could knock him out of football. Sure, the odds are that he'll be okay, but it would be a HUGE gamble for Miami if they head into the next draft without seeing Tua play. (Practice doesn't count, as I'm sure no one is allowed to clobber Tua in practice!) Suppose Tua sits all year, Miami uses those early draft picks on other positions, and then one big hit in next preseason puts Tua on IR or out of football permanently. They'll be without their franchise QB; the season might not be a bust but they'll be using a back-up QB all year, and they'll probably not be in a position to draft another top QB in the following draft. IF Tua is fragile, then they need to know that before this season ends. With extra early draft picks for next year, it's Miami's best chance at selecting a new "QB of the future" should Tua prove to be too brittle to play.
Heading into next season without seeing Tua take a hit would be a completely stupid move. IMO, cutting Rosen before knowing if Tua can take a hit was a stupid move. I'd expect idiotic decisions like this from Philbin and Gase, but I was hoping the Flores regime would be different. All Tua needs is a little bit of playing time. Let's see him take a few hits. Will he jump right up ready for the next play? Great! If he can't get back up, that would be a bad break for Tua, but the team would be in position to get a replacement in the 2021 draft. Giving Tua some playing time THIS season is the logical thing to do!