The win means that Rotherham have been relegated to League One, which means that Millwall are safe from the dreaded top from the Championship.
Millwall striker O’Brien has taken to Twitter to revel in his side retaining his Championship status for the 2019-20 campaign.
The 25-year-old has also thanked Rodriguez and Harper for scoring the goals for the Baggies against Rotherham.
West Brom are aiming to clinch promotion to the Premier League at the end of the season and will have to do it via the playoffs.
The Baggies are fourth in the Championship table at the moment with 80 points from 45 matches.
Leeds United, Aston Villa and Derby County are the other three sides in the playoff spots at the moment.
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