Thousands of people are expected to attend the funerals on Wednesday of Nevin Spence, 22, who along with his father Noel, 58, and brother Graham, 30, was overcome by slurry fumes at the family's farm near Hillsborough, Co. Down.
Muller said: "He was determined to be the best that he could be and I have no doubt that he would have gone on to play for Ireland on many occasions. He was the kind of player that inspired others with his attitude in training and sheer will to win in matches."
Health and safety experts are carrying out an investigation into the tragedy last Saturday afternoon.
Wednesday's funeral will take place at Ballynahinch Baptist Church, Co. Down and Ulster fans will pay their own respects at a memorial service at the team's ground at Ravenhill, Belfast on Sunday afternoon.
Muller said Nevin's loss was deeply felt by all the players.
He said in a statement: "Nevin Spence was a highly skilful and gifted rugby player. He was utterly committed to Ulster and was enormously proud to represent his Province.
"His positive outlook on life brought a smile to people's faces. He was kind, polite, generous and courageous, and we were blessed to have him in our team.
"The one comfort for us as a squad is that we have each other for support.
"It has been a difficult time for the players and coaches but I believe that through this tragic loss, we will have a new sense of togetherness."
- Nevin Spence