McIlroy noted he was also complimentary of Woods and Mickelson, who have combined to win 19 major championships and 121 PGA Tour titles. Neither, however, qualified for the Tour Championship for the first time since 1992.
"I was going through Twitter last night and it was like, 'Hold on here, did I say something bad here? I don't think I did.' ... I thought I was very complimentary."Rory McIlroy
"I was going through Twitter last night and it was like, 'Hold on here, did I say something bad here? I don't think I did,' " McIlroy said Thursday after an opening-round 69 at the Tour Championship. "I know Tiger and Phil really well. I get on really well with both guys. I've said much worse to Tiger before.
"I felt like I had to clarify what I said. I thought I was very complimentary. I said the only reason Tiger wasn't here was he didn't have the opportunity (due to injury). ... I wasn't writing him or Phil off at all. Phil withdrew last week. It was a long stretch of golf for him. He battles an arthritic condition. It's just what it is. I didn't say anything out of line or wrong.
"I was trying to be funny at the end saying we'll see how I feel in 20 years. I think you guys know how it was supposed to come out. I think it was taken out of context.''
During a Wednesday news conference preceding the Tour Championship at East Lake Golf Club, McIlroy was asked if the absence of Woods and Mickelson signaled a changing of the guard.
McIlroy replied: "Not really. Phil has played well in part this year. He came really close to winning the PGA. I feel like he's gotten better as the year has went on.
"But it's a lot of golf for him to play in such a short space of time. So you could see he was getting a little tired the last couple of weeks. And Tiger's not here just because he's been injured. He hasn't had the opportunity to play. But I think if he gets back and when he gets back to full fitness, you'll see him here again.
"They're just getting older. Phil's  or whatever he is and Tiger's nearly 40 (39 in December). So they're getting into the sort of last few holes of their career. And that's what happens. You get injured. Phil has to deal with an arthritic condition as well. So it obviously just gets harder as you get older. I'll be able to tell you in 20 years how it feels.''
McIlroy, 25, is tied for 11th, three strokes back of leaders Chris Kirk and Billy Horschel, who are also first and second in the FedEx Cup standings. The Tour Championship is the PGA Tour's final event of the season and the overall FedEx winner receives a $10-million bonus.