The three-time major winner had stem cell treatment on his left knee at the end of August after being bothered by a niggling injury for five months.He felt fine in his first outing of the new PGA Tour season in Las Vegas in October only to land hard on the knee after slipping on a wet piece of concrete during the CJ Cup in South Korea.Koepka, the leading qualifier for the US team, was then forced to withdraw from the Presidents Cup in Australia in December, missing out being part of winning side captained by Tiger Woods.Speaking as he prepared to make his return in this week's Abu Dhabi HSBC Championship, the 29-year-old said: "I've had problems with the knee since March, dealt with it the whole year and, after having the stem cell done, it felt fine all the way through to Korea. Everything felt good. "And then, in Korea, I re-tore it and the kneecap had moved into the fat pad. That's excruciating. It's a lot of pain. It's not fun."Koepka, who is returning to action with both Rory McIlroy and Jon Rahm breathing down his neck in the world rankings, has been busy doing rehab ever since and is now ready to test himself in the heat of competition in the first Rolex Series event of 2020 at Abu Dhabi Golf Club."I have been trying to make sure everything feels right, and it does not feel like my right knee, I'll be honest with you and it probably won't for a while," added the 2013 Scottish Hydro Challenge winner in Aviemore. "But it does feel stable, which leaving Korea and all the way up to about a month and a half ago, it felt like it could go either way. It could go left, out, back, it could go anyway."Even when I got the green light, I just didn't know whether it was going to feel right, whether you're going to be the same, how is it going to feel."I started hitting balls just before Christmas. Everything felt good. Speed was the same. We were hitting on TrackMan and my numbers were exactly what they were the day I left, which is always nice."From that moment on, after a couple days of hitting balls and not feeling pain, it was, okay, I could get back here and do this and finally play."