Montgomerie, one of Europe's top players, finished the final round of the German Linde Masters tournament Sunday and said he was in deep pain from his lower back problems.
Afterward, Montgomerie said he wouldn't play the Ryder Cup unless he could guarantee European captain Sam Torrance that his back would be fine for all the matches at The Belfry, England, starting Sept. 27.
'My gut feeling is I'm going, but I'll make a decision next week,' Montgomerie said. 'If I can't play five times for Sam, I'm not going.'
Montgomerie will play the American Express next week in Ireland, then make a decision.
Ryder Cup co-captain Ian Woosnam said Montgomerie's constant change of mind could hurt the European team.
'He says that all the time -- he'll be there,'' Woosnam said. 'It's not good for the team's preparations.
The Scotsman, six times Europe's leading money winner, had been optimistic during the first three rounds of the tournament about his back. Before the Linde Masters, he said he needed the event to test the injury before committing himself to play the Ryder Cup.
But after Sunday's final round of 5-under-par 67, ending a good tournament at 16-under 272, Montgomerie changed his tune -- although he was happy about his game. The 42-year-old is coming off a two-week layoff because of the back pain.
'The pain is tiring,' he said. 'Otherwise I came out with a top-10 result, so that was good. To be honest I was just practicing out there for the Ryder Cup.'
He is one of Europe's most experienced players, having played the Ryder Cup six times. England's Ian Poulter will replace him if he pulls out.
Full coverage of the 34th Ryder Cup