90 mustang problems..... need help.

[90 'stang]

I have a 1990 mustang lx 2.3 L, and today I went and got the oil changed. The mechanic told me that the rear main seal is leaking oil. I went under to look at it and sure enough it is leaking badly........It has sprayed from the seal (by the tranny and oil pan) all they way to the cataclytic converter. my whole undercarriage is covered with oil. The mechanic told me i need to get it fixed (duh) but also said that the leak could get on to the clutch plate and make my clutch start to slip. is this true? and i havent seen any oil leaks on the ground. and i have been looking. why wouldnt i see leaks?