.>The problem was that he claimed that he was Eugene's military mentor. <
How many times do I have to tell you that he did nothing of the sort? All he wrote about is appointment is this:
>It was, I believe, at the beginning of April that I received this order (dated 28 March 1809), at first I hid its nature from them. The minister did not explain to me in what capacity I should be going and he was unable to tell me much more himself; he showed me the original order.....thus dictated:
'Monsieur le duc de Feltre, give the order to general Macdonald to go to Italy where he will receive new orders from the viceroy and I shall be grateful to him for the services he will render. Napoleon.'<
He did claim to have given Eugene a lot of good advice and moral support, which is quite probable (there would have been no way of stopping him) , but he never claimed to have any role other than wing-commander.
Other saw it differently:
Captain Noel, With Napoleon's Guns: 'The army was under the command of Prince Eugene, so young and brave and so well loved and admired. He had fought battles before but had never previously been in command.......This battle of Sacile was a bloody one and the hasty retreat, in terrible weather, beyond the Piave, caused us many losses..... Happily General Macdonald arrived at that moment. He had been in disgrace for some time, I do not know why, and had been sent to help Prince Eugene with his advice. General Macdonald, in spite of his defeat at Trebbia, had made a favourable impression on the Army of Italy, and the soldiers, who had begun to be discouraged by the weakness shown by their commanding officer, regained confidence, for it is essential, as they say, that malcontents should be strictly disciplined. General Macdonald rejoined the army wearing his old Republican uniform. This gave pleasure to the older soldiers, but was turned to ridicule by some young fools and arrogant officers, of whom there were beginning to be far too many in the army and at headquarters.'
Givens Eugene's youth and inexperience - he was only in command because he was the Emperor's stepson after all - it was only natural for writers to assume that Macdonald was sent to supervise him, particularly after Thiers published his version, it wasn't Macdonald who said it!