PSG were among the many clubs trying to sign the teenager, but Alonso's confidence ensured Madrid won the race.
He told reporters: "I had already spoken to Xabi, he called me, it was very important for me that he showed that confidence. It was private, but his call gave me a lot of confidence, it meant a lot.
"Several people were interested in me, and I have the utmost respect for them. I spoke with Luis Enrique, and he was very clear with me. I congratulate him on winning the title last night."