Jerome Sinclair has returned to Watford after his loan spell was brought to an early end.
Sinclair has struggled for minutes recently and the three parties have decided to activate a break clause in the season-long deal.
Ross paid tribute to Sinclair's attitude during his time on Wearside and hopes the 22-year-old can kick on after 'taking a breath' at Watford.
The Black Cats boss is in the market for another orthodox striker this month and that has been a factor in the decision.
"Jerome has left today," Ross said.
"We had to collectively make a decision before Monday.
"He’s such a lovely boy, really really good.
"It just hasn’t clicked.
"We just collectively felt, he needs to play games and he obviously knows that we’re trying to strengthen that area of the pitch again.
"I think he’ll go back to Watford, take a breath and see what he wants to do next.
"I can’t speak highly enough about his attitude, he’s been at it every single day even when he’s been out of the team," Ross added.
"He’s not been one bit of bother.
"Everyone at the club will tell you he’s a really nice man, as I said, it just hasn’t quite clicked."
Sinclair's time on Wearside was disrupted by an early injury.
After coming at half time against Charlton Athletic on the opening day of the season, his performance was seen as a key factor in the Black Cats turning a one-goal deficit into a vital victory.
He picked up a knock in that cameo, however, and did not return to senior action until September.
Some promising performances followed, Sinclair scoring against Peterborough United and drawing special praise from Ross for his contribution in a win over Doncaster Rovers in October.
But his form dipped and with Josh Maja in sparkling form, opportunities became increasingly sparse.
The return of Charlie Wyke and Duncan Watmore only made his task harder in terms of breaking back into the first team picture.
The 22-year-old leaves with two goals in 19 appearances.