Smith, 49, said: “He's someone I felt we needed. I look at the Premier League and you need legs in your midfield area because you've got to cover a lot of ground in a game. Ross does that.
“He's got ambitions to get into the Euro squad at the end of the season.
“With the quality Chelsea have got they couldn't guarantee anyone minutes. His work ethic and attitude has been tremendous. He brings us that added bit of quality that we needed."