The young Scot was sidelined for three weeks in December after fracturing his cheek bone in a 3-1 win over QPR.
He has been influential for Watford this term and was central to their early season form in the Championship.
Since joining from St Mirren in January last year McGinn has made 42 appearances for the Hornets, scoring twice.
After the draw with Doncaster at the weekend boss Malky Mackay stated he would probably have to bring in a loan signing to help out his squad.
Mackay is already without defender Lloyd Doyley, while Andreas Weimann and Danny Drinkwater are both looking to shrug off knocks ahead of Saturday's visit of Millwall.
Captain John Eustace will miss the clash as he serves a one-match suspension.
Source: BBC Sport
Source: BBC Sport