There are inline adjusters etc., but considering paying about $10 for an adjuster, I'd just spring for a new RDER and avoid the hassle of inserting it inline.
Depending how the cable is seated in the broken adjuster, that may be a constant source of poor shifts, if the housing end is moving around.
Fix it right for around $30-35.
BTW, Acera is plenty good enough for 8 speed.
I have an Acera & Altus RDER's on my bikes that shift 9 speed fine.
Last edited by Bill Kapaun; 07-20-15 at 10:59 AM.