It prompted the Yorkshire club to splash out to take Sharp back to his boyhood club, just two seasons after selling him to Scunthorpe for £100,000.
He has yet to score in the league since his return, but saw a late goal disallowed in the 1-0 loss to Norwich on Saturday.
And although he was unable to prevent his side suffering defeat, Robson was pleased with Sharp's performance.
"Since Billy's been at the club that was the best bit of football for the first team so that impressed me," said Robson.
"That's what you want from your players. If you've been on the bench or in the squad you've got to try impress me when you get into the team."