McDonald made the $6 million move to English Championship side Middlesbrough to reunite with former Celtic boss Gordon Strachan in one of the biggest switches of the transfer window deadline day.

But his move was overshadowed by his replacement at Celtic when Mowbray signed Spurs hitman Keane on loan.

"The Robbie Keane move has taken me by surprise," he told Scotland's Daily Record. "Robbie's a great player and part of me thinks it would have been great to be at Celtic with him.

"Because of the type of person I am my first instinct is to think, 'We could have had a great partnership'. I think like that because I've never ever doubted my ability.

"Then again, maybe the manager would see things differently. Let's be honest, all these signings Celtic have made haven't just been planned overnight.

"The club must have been working on them for a while, so my decision to leave has probably been vindicated by the news Robbie is on his way up the road. I'm doing what is best for my own career."

McDonald admitted recent months at the club have been a struggle since Mowbray came in and started leaving the 27 year old on the bench, despite his impeccable record of 65 goals in 126 appearaces for Celtic.

As the transfer window loomed, McDonald realised he had to move on. Mowbray signed a string of new players in the January transfer window, including Keane and former Fulham striker Diomansy Kamara, while selling off even more.

"It didn't come as a shock," he said. "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."

McDonald added: "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.

"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."

Despite his enforced departure for the sake of his career - and possible place at the World Cup with Australia - McDonald insists he doesn;t bear any grudges against Mowbray.

"I could probably sum it up in one word - frustrating," he said. "It was nothing more than that and it was certainly never personal.

"The manager was fine to work with and he had decisions to make which obviously not everyone could agree with. To say I've left because of my relationship with the manager would not be correct. That was certainly not the only aspect.

"Let's just say, I didn't agree with some of the things he did and then, when the transfer window opened, I started to hear a lot of rumours about players he wanted to sign.

"The way things were going made me reassess my position and when Middlesbrough came in I realised I had to make a change. I was playing a lot of games but wasn't finishing many and it was increasingly frustrating.

"I didn't join Celtic to be a bit part player and that's the way I was heading."

He added: "I can't blame Tony Mowbray for that.

"He's the boss and he's got to do what he thinks is best for the team. These are the decisions he has to make and he'll live or die by them in the end."