The 19-16 defeat was Ospreys' second defeat to Scottish opposition in a row after losing 24-16 to Edinburgh the previous week.
And Rees believes his team's upcoming fixtures will now be decisive in determining whether or not they are able to secure a play-off spot come the end of the season.
"Looking at the next two weeks now, there's no mistaking how big a period this is," he admitted.
"It's still incredibly tight at the top now, and yes, we are playing catch-up having slipped out of the top four, but we've got a fantastic opportunity to put that right with Leinster and Munster coming to the Liberty Stadium over the next fortnight.
"There's no doubt these are going to be two key fixtures for us, but with those games, and Glasgow still to come to the Liberty as well, our destiny is very much in our own hands so we aren't going to be too downbeat.
"It's about doing things properly, being clinical and having the belief. A big Liberty crowd can really make a difference, can play its part in what we are trying to do this season.
"Expectation has grown outside the camp since the start of the season maybe, but internally, and among our supporters, we realise that we are still building, and these two home games coming up can possibly allow us to continue heading in the right way if everyone pulls together on and off the field."
But Rees will be disappointed to see his side fall from the summit of the PRO12 table after leading the pack for so long this season.
And the fact that Peter Horne's 75th minute penalty for Glasgow came so late on in Saturday's game will be even more frustrating for Rees in echoing Tommy Seymour's last-gasp try which sealed the Scottish side's victory as Scotstoun last season.
"There's a feeling of déjà vu about this one" Rees acknowledged.
"It's definitely a case of mixed feelings looking back at the game, but we'll try to take the positives out of it.
"The good thing is that we put together a game plan and everybody was reading the same page, the players stuck to it, and almost got their rewards."
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