Santos, 37, is set to play some part in the reserves' cup match against Leicester City this afternoon. The Millers become his ninth English club, nine years after he signed for Tranmere.
Santos, who has had spells at West Brom and Grimsby, is perhaps best known for his time at Sheffield United and in particular his role in the infamous Battle of Bramall Lane, which he sparked with a career threatening challenge on Andy Johnson (thought to be in retaliation for Johnson breaking Santos' cheek bone with an elbow in the previous season). Santos was one of three Blades sent off that day and the game ended up being abandoned after injuries forced the Blades down to six men.
This close season saw Santos without a club and he signed for Chesterfield on a short term contract basis over the summer. That contract expired on 1 November without him having made a first team appearance.