With the hosts 3-0 up in the series it was decided that all-rounder Woakes and slow left-armer Kerrigan would be given Test debuts but neither impressed as Watson cracked a Test-best 176 before being superbly caught by Kevin Pietersen just before stumps.
That saw day one draw to a close with Australia on 307 for four with Watson seemingly targeting the new bowlers, and Kerrigan in particular, as he helped himself to 25 fours and a six during his innings.
"I was expecting Tremlett to play to be honest. He had quite a lot of success against us in Australia in the last Ashes series," said Watson.
"He bowled beautifully in the games he came in so I was certainly expecting him to play, more so than Woakes and Kerrigan.
"In the end they have obviously got their reasons why they are picking those guys and I'm certainly a little bit happier than I would be having to face a guy who is six foot eight with balls bouncing up all of the time - I'm certainly not complaining."
The 32-year-old, batting at number three, had been waiting for almost three years to add to his previous two Test centuries and revealed going up against Kerrigan, who he had played so well in a tour match over the weekend, was a particular boon.
"Once the toss went up and I knew he (Kerrigan) was going to play I certainly understood what he was going to bowl to me," Watson said, scarcely concealing a smile.
"After the last tour match I certainly had a game-plan of what I was going to do and I followed on from that in this innings.
"You could definitely sense that he was very nervous, especially after the first couple of overs. There is no doubt that on your debut it is extremely nerve-racking, especially if someone gets after you while you are trying to find your feet."
James Anderson took two of the four wickets to fall to move second on England's all-time list with 326 scalps - ahead of Bob Willis and behind only Sir Ian Botham.
Despite that he described the pitch as "lifeless" and offered support to Woakes and Kerrigan.
The latter is a Lancashire team-mate of Anderson, who pointed out the success or failure of England's selection gamble could only be judged at the end of the match.
"It was a tough day for us but it was a pretty lifeless pitch to be honest," he said.
"It's difficult to say whether (this XI) worked after one day."
Yet there was no escaping the fact that 24-year-old Kerrigan had a chastening start to his international career, his eight overs costing 53 runs and containing a shocking number of full tosses and long hops.
Anderson accepted as much but believes the senior members of the side have a responsibility to support their new colleague behind the scenes.
"The guys in the dressing room have a job to do tonight. We have to rally round him and keep his spirits high," he said.
"He's had one bad day. He can come back tomorrow and come back strong, show everyone he can do it.
"Anyone can have an off day, not just a debutant...even if you've played 80 Tests you need people to help pick you up.
"The other 10 players know exactly what he can do, he's a quality bowler and he's been fantastic for Lancashire the last few years."
The last word belonged to Watson, though.
He finally had a reason to celebrate after a tough series that has forced him to question his own technique at times.
"It is a big relief to get a hundred there is no doubt about that," he said.
"For me the most important thing has been trying to bat for a long period of time. I have worked really hard and had some good people helping me out over the last six weeks to be able to get myself in to a place where I can give myself a chance to bat for a period like that. It is nice that it worked out today.
"I'm honest with myself and I know I have had a lot of things to work on, especially one thing in particular. I have done a lot of soul searching more than anything throughout this last month especially, more than at any other stage during my cricket career."
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