California natives can be difficult to get established in gardens, and many manzanita are particularly fussy. Far and away the best time to plant them is in the fall, when the rainy season is beginning. That way they receive enough water naturally (and you can supplement during dry stretches) to get established. Once established, they need very little water. Yours looks like it didn't get enough water when it was getting settled, which would make sense in a spring planting. You can cut off the branches that are completely dead and make sure that it gets enough water this winter. You almost can't overwater it in winter, since it is native to a wet winter climate. It might respond. Also, drip irrigation has a number of pluses, but it in no way resembles the heavy soaking that CA natives get in winter in the wild.

Soil immediately around it does look quite dry. Whatever the specific problem with that amount of brown, as well as browning foliage, extending all the way to the outside your shrub may already have been lost. As with ceanothus container grown manzanitas coming from nurseries are likely to be having been kept regularly watered while in production, same as any other potted stock not able to take suddenly and completely drying out, while sitting in the full sun. And named garden forms are likely to be more "garden tolerant" (able to take normal levels of watering, perhaps actually even needing it in some instances) than other material not originally selected specifically for garden use.