The 20-year-old central defender has joined The Robins for a two-week trial, after failing to break into the first-team setup at the Bescot Stadium.
Caines enjoyed a productive loan spell at Stafford Rangers last season but is now hoping to take his break in league football with Cheltenham.
Birmingham-born Caines joins former Wolves defender Graham Ward in trying to impress Robins boss John Ward enough to earn a professional contract.
However, it is expected Ward could seek a more experienced campaigner in his bid to replace Duff - who joined Burnley this week in a £30,000 deal.
