The situation is completely different than UNC. Players in the NFL are better, bigger, faster and stronger. He will have less time to throw and take more sacks. His value to UNC and his value to the Pats are not even in the same solar system. He's gone from UNC after a few years, but for the Pats he's the franchise for 12-15 years, if he pans out. Elite QBs are rare in the NFL. Even good QBs aren't drafted very often.
Finally, he's a young QB learning the NFL game. Do you really want to put him under that kind of duress, knowing that it could hamper his development, instill bad habits, ruin his confidence, and, in the worst case, cause a major injury? Why? So you can see him play? What's the value proposition? He's 21 years old. There's time. The Pats are likely going to suck this year, with or without him. TB12 didn't start until age 24. Let the kid develop and keep him safe until the line is solid.