The lowside of a casing joint is protected from wear while milling a casing exit for a lateral borehole. The casing joint is coupled to a casing string and is made of a material that is softer than that of the casing string. A whipstock assembly is arranged within the casing joint and has a deflector surface operable to direct a drilling assembly into a sidewall of the casing joint to create the casing exit. A wear sleeve is coupled to and extends axially from the whipstock assembly, the wear sleeve defining a throat that extends axially along the axial length of the wear sleeve and transitions into the deflector surface. The axial length of the wear sleeve extends beyond a point of contact where the drilling assembly would otherwise engage the lowside of the casing joint.