Gary Caldwell has revealed the unspoken fear of every top-class footballer in Scotland right now – that they run the risk of dying on the pitch, like Phil O’Donnell did last Saturday, and that they are putting their lives in danger in the pursuit of excellence on the field.

The confirmation yesterday that the cause of the Motherwell captain’s death was a heart attack has made O’Donnell the second high-profile footballer in Europe this season to die. Antonio Puerta, the 22-year-old Seville defender, who had arrhythmogenic right ventricular cardiomyopathy, died in August, three days after he suffered a series of cardiac arrests during a Spanish league game.

Caldwell believes there are too many modern sportsmen whose deaths are unexplained. The Celtic and Scotland defender, who publicly spoke last year of how he had lifted his fitness to new levels since moving to the Scottish champions, admits that everyone in the profession has seen an unwelcome glimpse of their own mortality by O’Donnell’s death.

“It would seem right now, that the fitter you are, sometimes a time bomb is just hanging over you, waiting to go off,” Caldwell said as he reflected on the death of the former Celtic midfield player, who was regarded as a supremely fit 35-year-old.

Caldwell called for research into the effects of food supplements and energy drinks and the way players refuel between games.

“There should be some study into these things,” he said. “I have thought about it. You drink energy drinks, you take supplements and you do it to become a better player. Every player wants to get the best out of themselves. But I think somebody has got to look into it and try to see if it does affect you. We don’t know enough about what other things can happen.

“It is no coincidence that there have been quite a few footballers who have died this year. I think there has to be better checks on players. In the United States, there have been a lot of big athletes dropping dead in recent years. Now we have it in football. It has to be looked at. Things like Phil’s death should not happen.”

In 2003, Marc-Vivien Foe, the Cameroon midfield player, died after suffering a heart attack during the Confederations Cup semi-final against Colombia in Lyons. In September 2006, Mohammed Wahab, a defender in Egypt died during training.

At 24, Caldwell is too young to be a contemporary of O’Donnell but he did grow up as a Celtic supporter watching the team that O’Donnell played in for the club, between 1994 and 1999 after his £1.75 million move from Motherwell. Caldwell insists that O’Donnell’s death has affected all footballers regardless of how close they were to the former Scotland international.

Caldwell and the rest of the Celtic team should have been facing Rangers today in front of 60,000 people. However, Gordon Strachan’s players will simply have a light training session and then return home to their families. “It should have been one of the most important games of the season, but it just did not matter to us.

“I don’t think I have ever come to a point in my life where football seemed as insignificant as it did on Saturday night when I heard the news. It does put football into the background and who wins leagues and who wins points does seem irrelevant at this time.

“We are all distraught and it’s a hard time for everyone at the minute. I think everyone’s thoughts are just with Phil’s family and Motherwell as well. We are just thinking about his family. God knows what they are going through. Hopefully, everyone can gather around them at this time.

“I didn’t know him all that well. He just seemed like a really nice guy. He was always smiling and happy and it’s tragic that he died so young.

“We did not want to play against Rangers but we were put in the situation by the powers-that-be [before the call-off] but we had to play, we would just have got the job done.”

O’Donnell’s name is added to a list of people who have lost their lives playing the sports they love. Sudden Death Syndrome is the umbrella term used for about ten cardiac conditions affecting heart muscles and the electrical activity of the heart. One of those was Daniel Yorath, the son of Terry Yorath, who was O’Donnell’s manager at Sheffield Wednesday. The 15-year-old, who had just signed for Leeds United, died in 1992 playing in the garden with his father.

An organisation called Cardiac Risk to the Young (CRY) was founded in 2002 by Alison Cox and her husband, Mark, the leading British tennis player in the Seventies. She has stated that pushing the body to physical extremes could “exacerbate an undetected abnormality”. When the Coxes’ son, Steven, an outstanding junior tennis player, went on a sports scholarship to an American college, they learnt – just in time – about his own heart risk. Steven Cox had to give up tennis.

Phil O’Donnell died of heart failure, his club said last night. The Motherwell captain collapsed as he was substituted during the game against Dundee United on Saturday. In a statement released last night, Dr Robert Liddle, the club doctor, said: “The post mortem revealed that Phil had suffered left ventricular failure of the heart.”

The funeral service for O’Donnell will take place in Hamilton on Friday at noon, the club added. The Requiem Mass, celebrated by Parish Priest Father Henry O’Brien, at St Mary’s Church, Cadzow Street, Hamilton, will be followed by internment in the nearby Bent Cemetery. At the request of his wife, Eileen, the church service will be for family and his many friends.

Following the internment, the family, friends and other guests will attend a reception at Fir Park.