McManus tore ankle ligaments 39 minutes into Middlesbrough's 2-1 home win over Crystal Palace on 6 November.
Up until that point the 28-year-old had played every minute of Middlesbrough's league campaign this season.
He has made 32 appearances for the club since an initial loan move from Celtic in January last season.
The transfer was finalised in July, with Boro paying £1.5m to secure the international defender on a three-year contract.
Source: BBC Sport
Source: BBC Sport