Monfils injured his left leg and was bandaged just below the knee when trailing 3-0 in the second set and despite two medical time-outs, the world No 13 lost much of his momentum.
Lopez cruised through the second set thanks to two service breaks and the tiebreaker proved an anti-climax with the top seed managing to take just one point against a rampant rival.
"The injury to Gael made it difficult for me as I was unsure whether he would continue. It is fantastic to be in a final as this is only my third tournament of the season," said the Spaniard.