Marsh learning first base wouldn't be out of the ordinary (Bellinger, Erstad, etc.), but I don't think it's necessary.
Pujols and La Stella are good options at first base, and it's too early to give up on Thiass.
Thiass hit .211 with 8 HRs in his first 147 AB, compared to Trout who hit .220 with 5 HRs in his first 123 AB.
Obviously Thiass will never be Trout, but what he has done so far is respectable.