Legendary Indianapolis Colts coach Howard Mudd -- the man responsible for getting O-linemen to block for Peyton Manning for YEARS -- died Wednesday, team owner Jim Irsay announced.

He was 78 years old.

According to Mudd's family members ... the famous NFL coach was involved in a serious accident on July 29 -- and succumbed to injuries Wednesday.

"Right after the accident he fought so hard against all odds to communicate to us that he loves us and that he knows we love him," Mudd's family said in a statement.

"[On Tuesday], it became clear that he was ready and that we needed to surround him with love and fight for his right to comfort and peace."

"This morning he was surrounded in the room by his sons [Darren and Adam] who held his arms and prayed over him as he passed away."

Mudd is known as one of the league's greatest offensive line coaches ever ... he famously produced O-lines year after year in Indy that protected Manning during the Hall of Fame QB's prime.

Mudd ultimately coached for nearly 5 DECADES in the league ... and was a damn good O-lineman himself during his playing days in the 1960s.

"Rest in peace, Howard Mudd," Irsay said Wednesday. "Howard was a GREAT player during a shortened career and then became one of the game's all-time greatest offensive line coaches."

"He contributed to many different teams over 47 years in our league---but he will always be a Colt. #LoveYouHoward."