The Iron initially said the 22-year-old had signed a deal until the end of the season with Iron having an option for a further year, but later clarified they had instead signed him on a one-month loan.
This is because under Fifa rules a player cannot represent more than two clubs in a season, but as this is an "emergency loan" he is eligible.
Walker joined Watford from Middlesbrough in 2010 and went on to make six appearances for the Hornets.
The midfielder has played in the Premier League for Boro and scored against Bayern Munich in the 2008 Uefa Cup during a loan spell at Aberdeen.