Midfielder facing four-match banShaun Williams took to Twitter to apologise after his red card in Sunday's home defeat by Championship leaders AFC Bournemouth - a dismissal that brings a four match ban for the midfielder.
Williams was shown a straight red card in the 79th-minute by referee David Coote for retaliation after Callum Wilson had reacted to the midfielder's initial foul.
Because the 26-year-old has already been sent off already this season - in the 3-3 draw with Wolves - he now faces an additional one-match suspension on top of the customary three game ban he would normally serve.
He said: "So disappointed in myself today it's not my nature I apologise to the lads and the fans for my reaction #illbeback"
Williams is now ruled out of Millwall's games against Bradford City, Blackpool, Ipswich and Reading and will be available again for the away trip to Nottingham Forest on 31st January.
However if The Lions have an FA Cup replay with Bradford City, Williams would finish his suspension in time for the Reading home game on 24th January.