The five-time major champion, back on the PGA Tour after a four-year hiatus with LIV Golf, posted a final-round 70 at Sedgefield Country Club on Sunday.
Koepka was tied for 70th when his round finished, way short of the fourth-place finish he needed to secure qualification for next week's St Jude Championship.
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"I struck the ball great," Koepka explained. "It's just all on the greens. I don't know what the deal is.
"Got zero confidence, can't putt. I don't know. There's going to be a lot of changes here soon."
Asked to sum up his disappointment at missing the playoffs, Koepka didn't hold back, adding: "Pretty disappointed. I think it's pretty pathetic that I can't get through.
"Zero top-10s basically in two years. To be in the prime of my career and be as good as I think I am, I think that's very pathetic, disappointing.
"I just need to go home and completely re-evaluate everything."
Koepka, 36, has not made a PGA Tour post-season appearance since 2021, having featured every year, except 2020 when he was injured, dating back to 2015.
The American has secured just one top-10 finish in the 16 tournaments he has played since returning to the PGA Tour from LIV Golf.
Watch the PGA Tour's FedEx Playoffs St Jude Championship live on Sky Sports Golf and Sky Sports Main Event from 7pm on Thursday. Get Sky Sports or stream golf with no contract through NOW.
