United beat Mourinho's Chelsea to sign the rookie England left-back but had to splash £27m to get him from Southampton.
Shaw is being paid around £100,000-a-week at Old Trafford while Mourinho signed Brazil international Filipe Luis, 28, instead.
But Mourinho - who yesterday completed the re-signing of Didier Drogba - claims paying the going rate for Shaw would have destroyed the Chelsea dressing room and risked the club breaking UEFA's Financial Fair Play rules.
"If we pay to a 19-year-old boy what we were being asked for, for Luke Shaw, we are dead," said Mourinho.
"We kill our stability with financial fair play. We kill the stability in our dressing room.
"Because when you pay that much to a 19-year-old kid - good player, fantastic player - the next day, we'd have players knocking on our door.
"They'd be saying, 'How is it possible? I play for this club 200 games and won this and that. How come a 19-year-old comes here and gets more money than I get?'
"It would kill immediately our balance and we don't allow that.
"Filipe's much less expensive. And a guy like Filipe Luis played for Brazil, won titles in Spain, won European competition, played Champions League semi-final - this guy is much cheaper than an English young lad.
"And is he good for our average wages.
"Sometimes you have to make decisions. I don't criticise the other clubs for paying it. They can pay what they want. I don't have any comment about it. But for my club we can say it would be very negative for us."
Free agent Drogba returned to Chelsea yesterday on a one-year-deal.
The Chelsea goalscoring legend said: "It was an easy decision - I couldn't turn down the opportunity to work with Jose again.
"Everyone knows the special relationship I have with this club and it has always felt like home to me."
