Injury-hit Nadal has not played a match since a shock defeat by outsider Lukas Rosol at Wimbledon last June but will test his suspect knee in Vina del Mar on Wednesday when he takes on Argentine Guido Pella or a qualifier in the claycourt event.
World No. 92 Pella is the kind of player usually trampled into the dirt by the 11-times grand slam champion but Murray believes Nadal could be vulnerable until he gets up to speed.
"He might not have the same (aura) straight away and you would expect people in the first few months while he is not back to his highest level and think they could possibly upset him," Murray told reporters at London's Queen's Club.
"But over time, and if Rafa wins his first three tournaments back that changes everything and people will be worrying about him again. People fearing you comes winning and confidence."
Despite just returning from a knee injury which still causes him pain, the public will be expecting Spamiard Nadal to get back in the groove quickly.
Murray, however, urged caution, although he expects Nadal to be challenging strongly for an eighth French Open title in June.
"The expectation will be high and people will expect him to do things straight away but it could take a number of weeks and months before he's back to his old self again," Murray said.
"But he's been practising for a month and half and was pressing for the Australian Open before he got sick.
"Providing his knee doesn't have a setback he will win some matches in south America and if he's physically okay by the French Open he will be in good shape."
Should Nadal, who has slipped to world number five, struggle initially for form and match-sharpness, Murray will emerge as a real title contender at Roland Garros.
"I believe I have a chance of winning the French Open," said Murray, whose best result there was a semi-final in 2011 when he lost to Nadal.
"But to do that I need to prepare and use every single day as best as I can. I have an opportunity there and I have a chance. Stranger things have happened in tennis than a player in the top four or five winning the French Open."
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