Lbell911, United has 94 737s; 64 737-300s and 30 737-500s. They've been planned to be phased out for several years now, just like the 727 pre-911.
The initial 15-20 are probably coming up on heavy checks and it makes sense to get rid of them. United could probably phase out all 737-500s and half the 737-300s tomorrow just to get to a point where the airline is properly manned.
There are a ton of United pilots who complain about the excessive hours that they have to fly monthly. United could reduce block hours significantly across all fleets and still be undermanned. And with the current wide range of hours that the company can build a pilot's schedule to (65 min to 89/95 max), they would not need to displace or furlough with a reduction in block hours. Furloughing and retraining are very expensive; I'm sure that the company will do their best to avoid any retraining.