"He came out with a lot of promise and just never really lived up to it," Sterling said on Triple M's Rush Hour show.
"It may be something to do with the way fullbacks gets hit in the modern game today, but I've got to say that James Tedesco and the like aren't that big.
"It may well be to do with the side that he played with as well, the Warriors.
"He never kicked on from that [a good first season] so to answer the question, I thought he was disappointing in the NRL."
After originally arriving in New Zealand on a big money three-year deal, the Warriors announced in April that Tomkins would be returning to Wigan at the end of the season, with Sydney Roosters and Kiwis No1 Roger Tuivasa-Sheck secured as his replacement.
Tomkins cited home sickness as the reason behind his early release.
The 26-year-old fell victim to another injury in his left knee and was forced from the field during the second-half of Sunday's demoralising 50-16 thrashing at the hands of Wests Tigers.
He may not feature in Warriors colours again after being overlooked for Sunday's final round away clash against the Canterbury Bulldogs, but assistant coach Andrew Webster said he will be given every chance to prove his fitness.
"Sam's battled with that knee for a while. We'll probably give him every chance [to play] but it's looking like unfortunately Sam might have played his last game for the Warriors which I'm sure he's going to be disappointed with and we're disappointed as well," said Webster.