Since you are eating a very low carb diet during the week, at the very least, you'll need to carb load 1xweek to replenish muscle glycogen. Yes, you'll end up retaining water, but that's all it is - water. If it helps, weigh yourself the morning before you carb load, then weigh yourself the following morning. You'll notice a weight gain (~2-4 lbs.), but when you gain weight that rapidly, it's water weight.
You can't gain fat that quickly! You'll lose the water weight in 2-3 days, and end up with a greater weight deficit by the time you carb load again. Just to give you an example, I did a cheat (included fat) on Sat before my big deadlift on Sunday. I went up 3 lbs. in weight, but after one day of lower carb intake, I'm already back down 1.5 lbs. I don't normally weight myself that frequently, but I'm just fascinated by how much you can fluctuate your water weight.
If you don't replenish your muscle glycogen, you're going to end up getting weaker and weaker in the gym. If you aren't able to put forth the energy in the gym, you're not going to be able to maintain your muscle mass while dieting.
Now that I've seen your stats, I can see where the numbers are coming from; looks like your trainer is using 10xbodyweight to come up with total daily calories. The problem is you're not a very big person, so that's why the calorie intake is so low. I personally would have you increase those calories a bit, but I'm going to defer to Mr. X on this one.