That's why in Monaco qualifying is so crucial. Not only is overtaking in the race extremely difficult, but the tires will degrade even worse than in the other circuits because of the low downforce/cornering speeds.
Monaco will be another weak spot for Mercedes. I don't think Jenson will do good there, either, given he's complaining about the front grip on his car making it understeer a lot. He'd need to work harder, degrading the tires more quickly. For Monaco oversteer works best, so given that's how Sebastian, Kimi, and Alonso have their car set up I think any of those three will win--provided they finish in the front row or just behind the front row in qualifying. I can't see Hamilton winning it, as he's never been particularly good in the circuit.