From what you have described, it sounds like your employer acted legally.
There is no law that prevents an employer from laying off an employee while that employee is on short term disability leave.
FMLA is a different story. Employers cannot discriminate or retaliate against someone who took or is on FMLA leave. Unfortunately, the Family and Medical Leave Act only applies to "any person [or business] engaged in commerce or in any industry or activity affecting commerce who employs 50 or more employees for each working day during each of 20 or more calendar workweeks in the current or preceding calendar year." 28 U.S.C. 2611(4). Since your former employer did not have 50 employees, the FMLA would not apply.
Utah does not have any state laws that provide broader protection than that of the FMLA. One thing you may consider is whether or not your back surgery and subsequent time off is indicative of a disability under the ADA / ADAAA. While short term "disabilies" generally do not qualify as protected disabilities under the ADA, it may be worth investigating this because, if you do fall under the ADA, you may have a claim for retaliation and/or discrimination based on your disability.
More information on disability discrimination is available at:
I hope this information is helpful.
My thoughts and comments are for general information only. The do not constitute legal advice and they should not relied upon as such. My thoughts and comments do not form an attorney-client relationship--I am not your attorney. Please seek the advice of a licensed attorney before taking any acts based upon my thoughts and comments or the comments of others.