Hidden bombs containing metal pins went off as the bus travelled past a hedge on the way to a Champions League quarter-final match in 2017.
Dortmund defender Marc Bartra underwent wrist surgery and a police officer was also treated for injuries.
Merino revealed he could have suffered a fatal injury in the attack, as he normally sat next to Bartra.
Years on: Mikel Merino
AFP/Getty
"I think I have an angel on my shoulder," he told The Players' Tribune.
"I’m actually fortunate to be alive at all. I was 20 and playing for Dortmund when three pipe bombs went off right next to our bus on the way to a Champions League match.
"A lot of young people probably would not even believe that it happened, or how bad it was, but it marked us all. There was a deafening noise, and then I remember the bus nearly tipping over.
"I looked behind me and saw blood on the floor. Smoke, screams. I remember being at the hospital later that night, waiting for Marc Bartra, who had his wrist badly cut up. The next day, someone told me that a huge piece of shrapnel was lodged in the headrest of the seat next to Marc. My usual seat.
"That day, the seat was empty. My legs were feeling a bit tight, and the seats in the front reclined. So I sat up there instead, and it may have saved my life.
"So yeah, I just have this belief that things happen for a reason."