USA captain Tom Watson has called on Tiger Woods to give him an honest assessment of his fitness as he continues to ponder the former world number one's Ryder Cup fate.

Woods has failed to qualify automatically for the event at Gleneagles, having spent the 2014 season either recovering from back surgery or struggling for form.

Watson has previously hinted that the 14-time Major winner would be an automatic wildcard pick, but his withdrawal from last week's WGC-Bridgestone Invitational and missing the cut at the US PGA Championship has raised doubts which the 64-year-old has admitted only Woods can answer.

"The answer can only come from Tiger. I am confident he will give us an honest answer," Watson told reporters.

"The biggest question is his health and right next to that it is how is he playing. Does his back allow him to make the type of swing he needs to get back into form?"

Watson, who is captaining for a second time, has to confirm his three wildcard selections on September 3.