Depends on a/c type but on your average twin jet, not a lot - just a modicum of rudder to keep straight and obviously not as much retardation as you'd have if both reversers were working.
All fair and well if one anticipates using the reversers and it is know one is deactivated or if one fails to deploy when selected. However, it is a different situation when a reverser deploys uncommanded. Factors such as in which phase of flight this occurs and how much time is available to correct the situation, if the situation is identified as such when time is critical, come to mind.
Green-dot