Ospreys, who currently sit top of the GUINNESS PRO12 table with seven wins from seven, conceded 50 points for the first time in their history when they went down 51-35.
With several players away on international duty or injured, Rees' team found themselves 41-0 down at the interval and he admits the result is an indictment of Ospreys' lack of cover.
"We've spoken about the brittleness of our squad for a while," said Rees.
"Maybe the depth isn't there. We are finding out lessons all the time about players.
"Last week, I was doing work with players and in the past that group would have comprised around 12 to 15, helping to prepare for the next league game.
"This week, I've had five training with me in that group.
"The rest are injured or have international commitments, and we haven't been able to offset that, as we have in the past, with extra squad depth. But that's the reality of the sort of numbers we have training."
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