"It has been tough," said the former Hibs player. "I knew from the start what I was coming here to, the competition for places. I'm not going to give up.
"I think I've done well when I've come on and played. I think I've started only two league games this season, so it's tough just coming on for 10 or 20 minutes and trying to make an impact. But I've just got to keep working hard and, hopefully, I'll get a chance.
"I can only do my best and give everything I've got when I come in to the games."
Hampered by the fact that he, unlike his selection rival Jan Vennegor of Hesselink, cost a mere pittance, Killen was handed a surprise start in the away Champions League defeat to Benfica.
In the weeks since, however, the New Zealand international has discovered the truth behind an old maxim; it's far easier to look good playing against the Old Firm than it is playing for them.
"Last season, I was one of the first to be put on the team sheet. Now I'm one of the first to be put on the bench," Killen sighs.
"I was playing week in, week out, and I was able to do what I do. Sometimes it's hard to make an impact coming off the bench. When I left Hibs, I knew the task I had ahead. I knew the competition for places was tough - I knew what I was coming to.
"I'm naturally a hard worker and I think by working hard my time will come. I'm training as if I were starting the game but that hasn't happened. My chance will come and, when it does, I need to be physically and mentally prepared."