Low hot water pressure in one fixture after water heater replacement
Sounds like you need to replace the mixing valve. This is a simple part to replace. This usually involves removing the handle and possibly the trim, then removing a retaining clip, and the pulling out the cartridge. Buy a new one and reverse the process.

Here's a good set of instructions for Moen valves: http://www.moen.com/consumer-support/installation-help/tutorial?id=moen0133