Alekxei Pino filed the lawsuit on Friday in Florida ... alleging he was helping assist a stranded motorist on Feb. 6 in Miami when Highsmith barreled into him in his 2021 Audi A5 at a high rate of speed.

Pino claims in the suit Highsmith was going over the roadway's legal speed limit before he hit him ... causing an above-the-knee amputation.

Pino also claims he suffered other significant injuries in the crash ... and has since undergone numerous medical procedures to address the damage.

Pino's attorney, Manuel Dobrinsky, said in a statement this week his client is actually still in the hospital recovering ... "with a very long road of rehabilitation ahead of him."

Pino and his lawyer are asking for unspecified damages but estimate it will be in excess of $50,000.

Pino's family, meanwhile, has also set up a GoFundMe, hoping to raise $100,000 to help him with the recovery process.

As we previously reported, cops said in a police report Highsmith was traveling 45 MPH in a 40 MPH zone before the accident a short time after the Heat beat the Orlando Magic at the nearby Kesaya Center.

Cops stated in the docs they believed drugs and alcohol were not involved ... though they did say Highsmith may have been distracted by unknown circumstances. They ultimately cited him for careless driving.

The Heat said in a statement following the crash that their "hearts go out to those who were injured."

Highsmith sat out of the Heat game the night following the wreck ... although he returned to the floor on Feb. 11 against the Celtics, and played 23 minutes.