Bell missed the meeting with SPFL Premiership club United at Ibrox after being knocked out in training by team-mate Seb Faure in the build-up.
He was desperate to feature in a match his side ended up losing 3-1 - but eventually bowed to expert advice from Light Blues medical staff.
And the 27-year-old goalie has told how suffering another concussion in the last-four fixture could have had tragic consequences for him.
Bell said: "It was a frustrating week for me. It felt like I had missed a cup final. I knew how important a game it was for the club.
"But you just need to respect the doctor's position. He's an expert in that field and I had to respect his decision.
"He had to play on the side of caution and stick with the guidelines. It was frustrating."
He added: "We were playing a small-sided game and I dived at Sebastien's feet. He hit a shot and the ball hit me on the chin. I was knocked out straight away.
"I was out for a few minutes and when I woke up the physio was in my face.
"I felt okay after it. I went to the hospital and got checked out and everything was fine. But I knew I was in for a battle to persuade the doc.
"I stayed here until 4pm on the Thursday, getting in the way and trying to persuade him.
"But the doctor had a decision to make and I respect that. I understand why there are guidelines in place.
"If you take a second concussion, it can be fatal. Doctors can't take a chance. I understand his decision."
Meanwhile, Bell has hit out at the flak his stand-in Steve Simonsen received in the aftermath of his slip-up against United.
The Englishman, who was at fault for the Tannadice club's third goal, closed down his Twitter account after being barraged with abuse.
Bell said: "Being at a club like Rangers, you are always open to criticism. Football is an opinionated game.
"But the criticism that Steve took online? I don't agree with that at all. People went too far with that.
"I understand everyone has an opinion, but we go out to play well and do our best for the club.
"Everyone is working hard and some days it doesn't happen. But the views of the coaching staff are the ones that matter."