The Australian international joined Boro in a £3.5million deal on Monday.
“It didn’t come as a shock,” he told the Daily Record.
“The day the manager said he wanted 10 new players was the day we all knew changes were coming.
“He certainly wasn’t joking, was he? There can be no excuses now if things don’t work out for him.”
“It has been a very emotional time for me. Since Middlesbrough put in their bid last week there has been a lot of back and forth between the clubs and my head has been in a spin. Now it’s all been completed I’m glad it’s done and I know where my future lies,” he added.
“I know I’m going to get the chance to play football down here under a manager who rates me and that’s the bottom line.
“But the truth is I leave Celtic with a heavy heart. It’s a special place and I’ll take some great memories with me. We won the title there and that was the best moment of my career. I wish I had won a lot more but at least I’ll always be able to look back on my time at the club as a success.
“In an ideal world, I’d have spent the rest of my career at Celtic. Leaving them after just two-and-a-half seasons was not my plan.
“But sometimes unforeseen circumstances crop up and you are forced to reconsider your position. That’s what has happened to me.”
