Implied in what way?
Aboard the Pegasus
, Riker supposedly was at the helm. Aboard the Potemkin
, he wore yellow, but whether that was Security or Engineering or something else, we weren't told. He was but a Lieutenant there, and was never credited with a high position as such, but was credited with having the ship perform a clever tactical maneuver, and with leading an away team. Perhaps a Tactical Officer of some sort, then? Or then Security or Engineering, merely with the wits and guts to suggest tactical maneuvers to his superiors? Aboard the Hood
, he was probably the XO, because he was in a position to tell the skipper not to beam down, just as later with Picard, but that's not really explicit. Perhaps he was actually the Security Chief?
Riker knows his way around machinery, but so does Picard, and we never get the impression Picard would have been an engineer.