Kirk was overlooked despite winning the Deutsche Bank Championship the day before the wild cards were announced, and went on to finish fourth behind Horschel in the Tour Championship on Sunday.
Watson, who captained the US to their last win on European soil in 1993, said: “In ‘93 I made my two picks after the PGA [Championship], six weeks before the Ryder Cup.
“Three weeks out is a logical place to make your final picks because, logistically, there are so many different things that go into it to get the players over there, get their clothing, get their families involved. It would be awfully difficult to make a decision the week before. From a time standpoint it would not make any sense.
“I texted Billy last week and said ‘You’re a day late but not a dollar short’ [a reference to the $10 million bonus Horschel won in Atlanta]. He was on my radar earlier in the year and I like his swing, his fundamentals and his attitude, but he just did not perform well enough to get on the team.”
Watson knows he will get the blame if the United States lose the Ryder Cup for the eighth time in the last ten contests, but added: “I’m prepared for that. I have some thick skin.
“I’m trying to make the best decisions I can at the time. If the players lose rather than win, I don’t care. I will take the blame. If and when we win the Ryder Cup, I will stand aside and give all the credit to the players and caddies. That’s where it’s due.”
Watson admits Paul McGinley’s side look stronger, but knows there will be added value if his players can beat the likes of world No.1 Rory McIlroy and cup talisman Ian Poulter.
“Without a question,” he added. “You knock off the big dog and that gives a boost to your team without a doubt.”