Liddell, who was released by Oldham at the end of last season, joined the Millers on Friday night and is determined to make the most of his chance by repaying Robins' faith.
He told the club's official website: "I would like to express my gratitude to the manager for allowing me the opportunity to play.
"I had a phone call from (assistant manager) John Breckin to see what I was doing and at the time I was training with Bradford Park Avenue.
"I haven't done much pre-season due to my family circumstances, but recently I have got the bit back between my teeth and I want to keep playing football.
"This month will give me the chance to keep improving my fitness and hopefully I can be of some use to the club."