'Mickelson was two up with two holes to go?' Watson asked. 'He bogeyed No. 17 and double-bogeyed No. 18?'
After arriving in Prince Edward Island following a visit to remote salmon fishing camp on New Brunswick's Restigouche River, Watson and Nicklaus were mostly in the dark about Mickelson's collapse and Geoff Ogilvy's victory in New York on Sunday.
The two golf greats, speaking before a charity golf event at Rodd Brudenell River Resort, were at first hesitant to criticize Mickelson and Colin Montgomerie, who also had a chance to win on the 18th hole.
But both were soon expressing surprise at the shot selections after hearing what happened. Mickelson hit only two fairways all day but still pulled a driver on the 18th tee. He promptly sliced his ball off the roof of a hospitality tent and into the Open's notoriously tough rough.
The two were even more amazed that Mickelson made matters worse by trying for the green on his second shot rather than getting the ball back on the fairway. Mickelson ended up hitting a tree and advancing the ball only 25 yards.
'He had an easy option to get back into play? Really?' Watson asked.
'Put the ball in play. Go play golf,' Nicklaus added.
On his third shot, Mickelson hit a towering iron that sailed left of the green and came down with such force that it plugged in the bunker.
Nicklaus, who won a record 18 majors and 83 tour titles, said he learned early in his career to not take undue risks when a tournament is on the line.
'You don't ever give up the end of a golf tournament,' he said, adding he learned that lesson in 1963 when he picked the wrong club on the last two holes of an event he was leading and finished bogey-bogey and lost. I didn't like that feeling and said I'd never do that again, and I never did.'
Watson won eight majors, including five British Opens. He acknowledged it's easy to second guess but said: 'What are you playing for? You're playing to win, not be a hero. The only person you have to beat is yourself -- and he beat himself. It sounds like he beat himself because he didn't play the percentage shot.'
Nicklaus and Watson are to play in the Legends of Golf, an event designed to raise money for the Heart and Stroke Foundation of Canada and promote golf in the province.
The 66-year-old Nicklaus said Monday's round marked only the 11th time he has swung a club since ending his tour career at last summer's British Open. When asked if he has any plans to play another tour event, he shook his head.
'Maybe if I get a new body,' he said. 'I've lost three inches of height. My vertebrae sit one on top of the other, so I just can't move. I have no ability to turn or do anything else.'