London - Nine-times major champion Gary Player believes Phil Mickelson faces a "monumental task" to achieve a clean sweep of major wins and said it would be "eating him alive" if he was perennial nearly-man Lee Westwood.
Mickelson, who became only the second left-hander to win the British Open when he triumphed at Muirfield on Sunday, has set his sights on joining Player, Gene Sarazen, Ben Hogan, Jack Nicklaus and Tiger Woods by landing the full set of all four of golf's top prizes.
The 43-year-old American has five majors to his name, having also captured three U.S. Masters crowns and one U.S. PGA Championship, and needs a first win in the U.S. Open to complete a career sweep.
"That is going to be the biggest challenge of all the majors for him," South African Player told the BBC.
"Because he is not a good driver and when you play the U.S. Open you will have to use your driver.
"This is a monumental task for him but he has got that thing called 'it', which the majority of people don't have.
"I think he will probably win it.
"You can't describe it," added the super-fit 77-year-old who attended last week's British Open and still looks as lean and trim as he did in his heyday.
"It is a gift that is loaned to you from the man above."
Mickelson said a U.S. Open triumph would cement his status as one of the greats.
"If I am able to make it a career grand slam, I think that's the sign of the complete great player," he said after emulating fellow left-hander Bob Charles's 1963 British Open victory at Lytham.
"I am one leg away and it's been a tough leg.
"There are five players who have done that and they are the greats of the game - you look at them in a different light."
Mickelson won the U.S. Masters in 2004, 2006 and 2010 and the U.S. PGA Championship in 2005.
He said on Sunday the key to his three-stroke win over Swede Henrik Stenson in the third major of the year was his birdie at the par-three 13th.
"I was behind the whole day and one-over-par for the championship," explained Mickelson.
"I hit a really good five-iron at the 13th to 10 feet and I knew that was a putt that was going to make the rest of the round go one way or the other.
"It went in and it gave me a nice momentum boost because it's very hard to make birdies out here.
"You're not going to hit it to tap-in distance, you're going to have to make some putts.
"That was a critical putt because I made another birdie at 14, made some good pars at 15 and 16 and when I was walking up 17, that was the moment when I had to compose myself because there I hit the best two three-woods I've ever hit," said Mickelson.
"That is why I don't have a driver in the bag, those two three-woods were my best two shots of the week and got me on the green.
"As I walked up to the green, that was when I realised this is very much a championship in my control."
Westwood looked the likeliest winner of the British Open when he went into the final round holding a two-stroke lead over the field.
The 40-year-old Briton, however, wasted a glorious opportunity to end his wait for a breakthrough major victory at the 62nd attempt as he slumped to a closing 75.
"He has to be bitterly disappointed," said Player.
"He was in the perfect position to win the Open, over the last five years he has been the best striker in the game from tee to green.
"I said to my family if he hits the greens and fairways he will win but he was missing greens with eight-irons, he was hitting the worse shots I've ever seen him hit.
"It was one of those days," said Player.
"It would be eating me alive but I am different."