The 29-year-old, who recently had a trial spell with the League One side, initially left the Millers to join Crawley Town in 2010 before spending last season with Macclesfield Town.
Manager Steve Evans told the club's website: "I had not spoken to Pablo in a while but he rang me and asked if he could train with us.
"He came in, he trained, he played and let me put it into perspective, he has been simply stunning. He has been outstanding, he is fit, he is hungry and he is eager so we have brought him in until January.
"With Claude Davis being ill the other night we knew that we needed somebody. We looked at all sorts of players and different options, but we wanted someone to come in where we knew what we would get."
Rotherham face Notts County tomorrow.