QPR boss Harry Redknapp is interested in bringing Norwich midfielder Leroy Fer to Loftus Road.
Birmingham City have signed teenage forward Denny Johnstone from Celtic on a three-year contract.
The 29-year-old will be reunited with Millers manager Steve Evans, who was his boss at Crawley.
"A lot of hard work has gone into getting Matt Tubbs here, I am delighted," Evans told the club website.
"At the end of last season and before then I highlighted to my chairman he was my first-choice striker to be recruited, we have managed to get that done and accordingly I thank my chairman Tony Stewart."