Mumbai make two changes to their squad just like that, post getting knocked out of the season? Kumar Kartikeya and now Tristan Stubbs? It looks like they're trying to make use of the loophole that allows them to retain the player who comes in as an injury replacement. This means next year, Mills will be released into auction and Stubbs will be retained.
The reason I say this is, Stubbs is a big hitting batsman in the Pollard/David mould, while Mills is a specialist bowler. The timing of this replacement (after their season is practically over) is dodgy. Similar thing with Kartikeya where a wrist spinner replaces a left arm medium fast bowler.