The Leeds United left-back won't play again this season.
Douglas battled through injury to see out the match after Leeds had made all three of their permitted substitutions - of which he was one.
And the Scottish left-back even managed to create Luke Ayling's equaliser as the Whites secured a return to the Championship's top two.
It was revealed this week, however, that as a result Douglas will miss the rest of the campaign with ruptured knee ligaments.
And discussing the £3 million summer signing from Wolverhampton Wanderers on Thursday, Bielsa told Leeds Live: "Douglas is not going to be with us because of the injury. We say sorry because we are not going to have him with us. We appreciate he was a very brave player in the last game. He had a serious injury, but carried on playing. That is something we will not forget about his behaviour."
The player Douglas replaced against Millwall, Ezgjan Alioski, is expected to return at Birmingham City on Saturday.
Explaining the Macedonian's half-time withdrawal, Bielsa said: "Alioski had played two games in the international break. His physical performance, distance covered and attacks and recoveries, was the same as always, but he was not quite as powerful in his speed. For Alioski to have this level is an advantage for us. It was the reason why we changed the player Douglas (gave) us a solution the team need(ed)."
Which players would make your Leeds XI to play Birmingham?
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