Sundowns were trailing 2-0 at half-time and were also down to 10 men after Denis Onyango was shown a red card for a last-man professional foul on Lerato Lamola.
Clinton Larsen's Celtic were tactically disciplined and expertly sat back and waited to hit the Brazilians on the counter-attack as they made the most of the space Sundowns left at the back.
MATCH REPORT: Celtic smash five past Sundowns
Mosimane explained his thinking after the match:
"A complete day off, two terrible mistakes in the first half, the send-off of Denis Onyango, and a team that's willing to wait and wait [to counter-attack], and I don't blame them because they are playing away, why not…
"But there's two things; it's either you opt to take a 2-0, it's easy for me and for the team, but we are Sundowns we have to play, we've got people over there that came to watch us, it's easy to say 'so be it' and hope for a 2-1 or a corner-kick or whatever, but it's in the culture of Sundowns, we must to play, and we have to be gentlemen about it and be able to accept that these things happen to all the big football teams in the world, it's not only Sundowns.
"But to be honest we have to apologise to out people for this kind of performance, five zero is not good for the image of the club , but i can tell you that even real acrid has got it, man its got it, arsenal got it, man city got it, so who are we not to get it? I'm not saying it acceptable, its unacceptable but it's a game of football these things happen in football.
"It will take the character of any individual in Sundowns, how do we behave as players, coaches, everybody to say when the chips are down, when you're on a low, low, low like this, we as coaches have to bring up the team and encourage the guys.
"It's a test of character to all the individuals at the club, if we are able to succumb and take the pain, it's very terrible but we have to take it and we have to apologise to our people, it's unacceptable.
"We played from the beginning, even up to the end we played, and that's Sundowns. We tried. We pushed the ball, we had the ball. We can't sit back and say let's take 2-0 and go home and hide behind two zero. It's football, it does happen.
Mosimane continued: "I have a very good team that plays good football, okay. I know it doesn't help now when you lose five zero, its like you make an excuse but so be it. If anybody thinks it's an excuse then so be it.
"We accept, we lost. There's no referee that made us lose the game, nothing. We lost, okay. Sorry to Masandawana."