Book that flight on AA
AAs has comparable size limits (19" x 13" x 9", no weight limit). Sherpas are collapsible/sqeezable and AA even states in their policy they accept slightly larger-than-regulation carriers should they be collapsible. You definitely can get away with a large Sherpa on AA, seen it multiple times.

As for US. I don't know. If there's no more feedback in this rarely-visited forum, I'd ask this again in the US pre-merger forum.
And avoid the flights operated by the E175 regional jet except when in F or Row 8.