The Wales international, 22, is a free agent having been released by Bristol City this week.
"Christian Ribeiro was well paid. I spoke to his agent [prior to his release] and they were waiting for an outcome," said Knill, speaking at the Fans Forum held at the club by BBC Radio Humberside on Wednesday evening.
"There will be a lot of suitors for Chrisitana nd if we can do something we might. A lot of it is now down to the player, so we will just have to wait and see."
Ribeiro made 10 appearances on loan for the Iron after joining on loan in January, before returning in March.