In 1858, Morphy faced this move once against Johann Jacob Lowenthal before playing it twice in his match with Adolf Anderssen. He also played the move in games against Thomas Barnes and Jules Arnous de Riviere. Morphy would have been familiar with this move before his trip to Europe because it had been played by Charles Henry Stanley against Eugene Rousseau in the match in New Orleans in 1845.
The move 3...a6 in the Spanish first appears in the writing of Domenico Ercole del Rio (1750). His brief work was absorbed in a longer work by Giambatista Lolli (1763). Lolli's version then served as the basis for part of John Cochran, A Treatise on the Game of Chess (London, 1822). Cochran's Treatise point out that Ruy Lopez had suggested 3.Bb5 as a refutation of 2...Nc6, preferring to defend the pawn with 2...d6 and then offers, "how far he has succeeded in proving the move to be bad, the reader may judge by this game and the following Variation" (162).
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bxc6 dxc6 5.Nxe5 Qd4
White to move
This move is asserted by del Rio to be best.
6...Bxg4 7.Qxg4 Nf6
And the second player has "by no means an inferior game" (162).
In the variation, 3...Bc5 is offered instead of 3...a6. After a few moves, Black's position is equal or better in the lines given.