A lot of Black's problems in this game stem from his queen's knight. It goes to a5 on move 9, as is common in the Ruy Lopez. It then stays there until move 31, when it hobbles back to the poor square b7. Spassky immediately plays 32.b4 to stop the knight's possible re-entry via the c5 square.

From then on the poor beast stays out of the game until it pointlessly commits suicide on move 53. In the meantime, the knight became a target and there was no good way (and never enough time) to rehabilitate it. Moving to d8 would have at times blocked communication between the Black rooks, which could have caused problems, and even then, the e6 and c6 squares were not available.