Achilles tears are always serious, but they get even harder to rehab as an athlete gets older, according to Mike ... and Rodgers will soon celebrate his 40th birthday (December).

"To have it at the age of 39 is really rough because it makes the road to recovery that much more difficult," Dr. Mike told TMZ Sports' Mojo Muhtadi.

Mike says the average recovery time is 10-12 months ... but there are instances where recovery has been much quicker (up to 6 months -- this is very uncommon), as well as longer.

It's not a rare injury ... in fact, Mike says around 1 in 4 athletes sustain an injury to their Achilles, and it's not something that only affects football players.

"We even saw this injury with a late Kobe Bryant when he had this injury and also in his mid-30s. And he came back from it, but he was never quite the same. He didn't have that explosiveness," Mike explained.

Assuming Rodgers wants to continue playing, there's some question about how effective he could be as a player.

"I think it's safe to say his mobility will be limited even if he does come back. But the explosiveness is what suffers because it's hard to regain that strength. Although this day and age, we have some pretty cool technologies to help athletes heal faster and better than we did even 10 years ago."

And, Mike is pumped to see what the most modern medical tech can do for Rodgers.

"I'm excited to see the protocol that Aaron Rodgers goes through on this because I think it's going to be not only exciting as a sports fan but also as a doctor to see how the technology helps someone of his stature come back."

Bottom line -- Rodgers is one of the most talented and determined athletes to ever put on a helmet, and if he's intent on returning, Dr. Mike isn't going to count him out.