I agree about playing the best guy in most cases, but he would be dealing with linemen all season. Coming straight out of HS that's a tough row to hoe for 12-13 games. My concern with a true freshman linebacker is durability in the long haul. In college you see linemen that are intent on, and capable of taking you out on every play. Plus they're over 50 pounds heavier than you are, stronger, and many nearly as fast. In HS you may see a handful of those guys a season if you play in a quality league. In college, the opposition is stacked three deep with those kind of guys, all chomping at the bit to get at a true freshman on the other side of the ball.
With DB's and WR's it's different, they don't take the pounding that linemen and LB's do, nor are the guys they deal with play in and play out anywhere near as big or strong.
I'm old fashioned. Linemen and LB's should redshirt , IMO. The positions are too tough on the body and there's a huge difference between a 23 y.o. and an 18 y.o. in physical maturity and strength.
You're right, his injury is a done deal. I honestly think it's a blessing in disguise, though I'm sure it's disappointing for him. Knee injuries aren't the career killers they were 25 years ago. Rehab, put some quality muscle on, watch tape, and get ready for next year. I think it could be a big one for him.