The Englishman turns 40 later this month and Montgomerie feels his chances of landing one of the game's big prizes are diminishing rapidly.
Westwood took the decision to move himself and his family to Florida and compete full-time on the PGA Tour this season as he bids to enhance his preparation for the major championships.
He will find out this week whether that decision has paid dividends as he tees it up in the year's first major, the Masters.
Westwood has a fine recent record at Augusta National with top-three finishes in two of the last three years - his runner-up finish behind Phil Mickelson in 2010 a particularly bitter pill to swallow.
However, Montgomerie feels this year could represent his last realistic chance of claiming a Green Jacket, insisting that increasing pressure and the march of old father time will see him struggle to break through.
"I think this year will be his last chance - there is pressure now," the Scot told the Daily Mirror.
"I think England especially and a lot of Britain would love to see Lee Westwood win a major.
"He has moved to America to try to get this elusive major that we all want but at the same time putting pressure on yourself in this game is not easy. Very few players win or get better, improve, in their 40s, very few."
Montgomerie also believes that Westwood's well-documented short-game problems have held him back from achieving major success and fears they could again undermine his chances this week if he gets in contention once again.
"If you are asking if his short game is at the same level as Woods or Mickelson, no it's not," he concluded.