UEFA expelled the Polish champions from the competition after the ineligible Bartosz Bereszynski appeared in the 2-0 second leg victory over Celtic on Wednesday in the third qualifying round.
That decision meant Celtic, who were beaten 6-1 on aggregate against Legia, were awarded a 3-0 default victory and will now face Slovenian side Maribor in the play-offs.
But former Rangers defender Berg has slammed UEFA's decision and promised to challenge the judgement.
Berg told Sky Sports.
A row erupted when UEFA ruled that Bereszynski had not served a three-match ban for his sending off against Greek side Apollon in last season's Europa League.
This was despite the Polish international sitting out Legia's second-round qualifying tie with St. Patrick's and the first leg against Celtic. UEFA claim he was not registered against the Irish side and the ban was not fulfilled.
Berg believes that the punishment is unduly tough for what he believes to be an administrative oversight.
He said:
The Norwegian says there is a precedent in place for dealing with unregistered players dating back to Hungarian side Debrecen, and has urged UEFA to show consistency.
he added.