Miguel was on EPO big time, and I think I kind of proved it :
(comparing datas of the Lac de Vassivière ITTs in 1990 and 1995)
"In 1990, he was on his way to his first Tour de France top ten at general classification (10th). Some were saying he could have had a shot at winning if he had not sacrificed for his leader Pedro Delgado. He was the only one able to follow Greg LeMond to the top of Luz Ardiden, beating him for the stage win. IF THAT ISN’T GOOD DATA I DON’T KNOW WHAT IS ??? So where was Mr Indurain in 1990, let’s see… Mmmmhhh… Well, well, well : he finished 4th of the stage, just ahead of Greg LeMond, 40 seconds behind Breukink which gives him a 1h03’20” time.
Wait. Is that correct ? I see Mr Indurain did some training during the 90’s, didn’t he ? He managed to improve his own performance for a staggering 5 minutes and 46 seconds (9.1%) ! Well done, Mr Indurain, you’re a hell of a rider… and you sure helped cycling ride straight into hell. With his 1990 time, Mig-Hell would have finished 35th in 1995, just behind time trial specialist and clean sport poster boy Marco Pantani. A reference. Kind of."
https://greglemondfans.wordpress.com/2014/05/03/dark-side-of-the-lake/
That being said, Indurain was always a good rider; a phenomenal time trialist, especially. He won the 1989 Paris-Nice and Criterium International (where he did beat none other than Mottet, Fignon, Lemond & Roche fighting for the win).
I believe he had a first taste at EPO in 1991, saw that it worked, became much more aggressive by 1992 (docs were probably more efficient in dosage too).
I don't think he would have won the 1990 TDF if he had not sacrificed for Delgado. He would have done better than Delgado, but he would not have won. I also believe I read somewhere that he had to be convinced to actually believe in himself as a TDF winner. Maybe Echevarri told him that he would become one with EPO ? Can't say.