Pack up the tin foil hats and UFO paraphernalia ... NFL agent Leigh Steinberg tells TMZ Sports the conspiracy theory that owners blackballed Shedeur Sanders during last weekend's draft is "nonsense."

The former Univ. of Colorado signal-caller, of course, was largely expected to be a first-round pick ... but tumbled all the way to the fifth round on Saturday -- which had people racing to social media to accuse franchises of colluding with one another to keep Deion's son down.

Steinberg, though, told us this week there's just no way that actually happened.

"Because teams want to add to their roster players that give them a chance to win," he explained. "And that desire to have the best players overruns any other conspiracy theory. Teams would draft a player who's got great talent on the field -- he just didn't find a match in the process."

Instead, Steinberg -- Patrick Mahomes' agent -- said if people want to point the finger somewhere ... they should start with Shedeur's pre-draft process.

Steinberg told us he believes not throwing at the combine -- plus not preparing sufficiently for interviews with general managers -- were big reasons for his slide.

He also said the QB's 13-12 record with the Buffaloes -- and his famous father's large shadow -- likely played a role as well.

Nonetheless, Steinberg told us it could all be "a really good thing" for the young Sanders' career -- pointing out there's now no pressure for the 23-year-old to start right away.