Rovers will complete the signing of goalkeeper Stephen Bywater until the end of the season on Friday.
Paul Dickov lined former Derby County teammate Bywater up after the surprise recall of on loan Sam Johnstone by Manchester United earlier this week.
Bywater was released from his contract by Millwall - for whom he has not played this season - on Wednesday, paving the way for his switch to the Keepmoat.
Rovers goalkeeping coach Paul Gerrard said of Bywater earlier this week: “I have worked with this guy in the past.
“His personality and experience of winning promotion at this level is very, very valuable.
“To get someone with his experience is something that we are excited about and myself and the squad can’t wait to work with him.”
The 33-year-old spent the first three months of the season on loan at Gillingham after finding himself behind David Forde in the reckoning at the New Den.
Before joining the Lions, Bywater spent two seasons at Sheffield Wednesday, helping them to automatic promotion from League One, before being edged out by Chris Kirkland.
Starting out with Rochdale, the Oldham-born keeper switched to West Ham in 1998 for £300,000.
Eight years later - after loan moves to Wycombe, Hull, Wolves, Coventry and Cardiff - Bywater joined Derby for £225,000, going on to make 166 appearances for the Rams.