The contest, which was Spain s penultimate game before the Euros, ended in a goalless draw.
Llorente missed United's final game of the Premier League season at home to West Brom having been given early leave by Whites head coach Marcelo Bielsa ahead of his bid to shine at this summer's Euros.
Bielsa did not want to run the risk of Llorente suffering a recurrence of a muscular injury having come off during the interval of United's penultimate game of the season at Southampton.
Llorente had started his 13th Whites game in a row in the clash at St Mary's and his form upon returning from earlier injury troubles put him in boss Luis Enrique's squad for the Euros.
The Whites centre-back was named on the bench for the Friday night friendly against Portugal as Manchester City's France born-defender Aymeric Laporte finally made his full international debut for Spain.
Laporte, 27, has represented France at youth levels up to the under 21s and has received call ups to the senior squad but had yet to be handed his senior international debut.
The defender was also eligible to represent Spain, being of Basque descent through his great-grandparents and having carved out his early career in the country for Athletic Bilbao.
Laporte has been given Spanish citizenship and has switched allegiances to be part of a Spain squad missing a big name centre back in Sergio Ramos who was omitted from Enrique's squad for the Euros.
Laporte partnered Villareal's Pau Torres at the heart of Spain's defence against Portugal as Barcelona's Sergio Busquets lined up in the holding midfield role which Llorente can also fulfil.
Boss Enrique then made a change to his centre-back pairing in the 62nd minute but Lloernte stayed on the bench as Manchester City's Eric Garcia replaced Torres.
But Whites centre back Llorente was finally introduced in the 79th minute as Laporte made way.
Spain were lucky moments later when Ronaldo was played in only to see the chance wasted as a heavy header was intercepted by 'keeper Unai Simon.
At the other end, Llorente was firmly in the mix as Spain almost went ahead through a set piece which was saved by Portugal keeper Rui Patricio.
The Whites defender then produced a huge booted clearance to mop up the danger as Portugal again pressed but the game ended goalless as Spain's Alvaro Morata hit the crossbar in added time.
United's record signing Rodrigo has not been selected for Spain's Euros squad and was not involved in the friendly against Portugal.
Spain will sign off for the Euros with Tuesday evening's friendly against Lithuania.
Youngster Niall Huggins started for Wales under-21s in their Euros Under-21s qualifier against Moldova.
Huggins was substituted after 63 minutes with the score goalless.
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Thank you Laura Collins