Former Osasuna and Celta Vigo midfielder Angel Miguel Merino says Mikel always had the ability to score regularly.
"He had it in him, it was a matter of bringing it out and putting him in that position," he told El Larguero.
"It's not where he feels most comfortable because he likes to touch the ball more often, but when he has to finish, he's always been a player who can score. He's a good finisher and he's proving it."
He added, "I felt the same joy with the third goal as with the previous two, probably a little more because Mikel scored, but with the peace of mind that the second leg is still to come and we shouldn't get carried away.
"Real Madrid has shown on countless occasions that it can turn the tie around. The result gives you room to manoeuvre, but we know that matches and knockout rounds at the Bernabeu can't be considered won."
Merino has eight goals for Arsenal this season, six since he was switched to a striker's role in February.