In all liquid rockets, you've got to inject the fuel into the combustion chamber at a higher pressure than the combustion chamber itself. This is why turbopumps may be used--to raise the pressure behind the injector to higher than the chamber--otherwise the contents of the combustion chamber would flow backwards.
In broad terms, about what velocity do the propellants come out of the injector plate? And does it contribute to the total thrust of the engine in any significant way?
Tim Dodd (update: not Tim Dodd) asked a very similar question not too long ago. However, the answers are almost completely unsourced. Based on the Stack Exchange blog, I think this slightly differently worded question is still worth asking, just in the case it gets some sourced answers.