The German team has yet to finish higher than third in Formula 1 this year, despite claiming three pole positions, after struggling with tyre management.
But the Ferrari driver believes that if Mercedes can take pole position in Monte Carlo, it will be favourite for victory.
"Mercedes will be strong," said Alonso.
"They have been on pole position for the last three grands prix so we expect them to be very strong again.
"They were on pole position last year with Michael [Schumacher] before he got the [grid] penalty.
"We know how difficult it is to overtake, if they are on pole position they are favourites."
Alonso added that he is not too concerned if Mercedes does finish ahead, provided he is able to outscore his world championship rivals.
The Spaniard is 17 points behind world championship leader Sebastian Vettel, with Lotus driver Kimi Raikkonen 13 clear of him.
"We are not going to forget our main target, which is the world championship," said Alonso.
"At the moment, Vettel and Raikkonen are ahead of us.
"If Mercedes has to win one race, this is the one and we just need to concentrate on finishing in front of the others."
ALONSO EYES MONACO TRIPLE
Despite focusing on the world championship fight, Alonso is determined to end his Monaco GP victory drought.
He won the race in 2006 for Renault, adding a second victory for McLaren a year later (pictured), but has not managed to do better than second since.
Ferrari has also struggled in recent years, taking the last of its eight Monaco F1 wins 12 years ago.
Asked if those winless runs gave him extra motivation, Alonso replied: "Yes, definitely. Obviously we want to win the championship as we said many times but Monaco is a special race.
"Monaco is the most important race in the championship.
"I can be the first man to win for three different teams so it is good motivation for me."
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