United States is without a permanent manager since Bruce Arena's resignation after its failure to qualify for next year's FIFA World Cup, but interim boss David Sarachan has named an experimental group to face the European champions in a friendly on 15 November.
After making seven Bundesliga appearances for Schalke this season, 19-year-old McKennie is handed a first senior call-up - as are Tyler Adams, an impressive performer for New York Red Bulls, and Josh Sargent, who has agreed a move to Werder Bremen in February.
Cameron Carter-Vickers - on loan at Sheffield United from Tottenham - returns to the squad as he looks to earn a first cap, while regular squad member Jesse Gonzalez is similarly awaiting a debut.
A number of other fringe players are recalled for the clash in Leiria as Sarachan leaves out those still involved in the MLS Cup play-offs.
"The one word that I would use in reference to all of this is opportunity," Sarachan said.
"It's an opportunity for many players who haven't been in the picture that we feel have a bright future with the national team to get to measure themselves in a game against a quality opponent.
"It's an opportunity for our national team to finish out 2017 in a positive way. It's also just an opportunity to move on. As much as we're still gutted from how things turned out with qualifying, we have to look ahead and finish out the year the right way."
United States squad in full:
Jesse Gonzalez (FC Dallas), Bill Hamid (DC United), Ethan Horvath (Club Brugge); John Brooks (Wolfsburg), Cameron Carter-Vickers (Sheffield United), Eric Lichaj (Nottingham Forest), Matt Miazga (Vitesse), Tim Ream (Fulham), Jorge Villafana (Santos Laguna), DeAndre Yedlin (Newcastle United); Kellyn Acosta (FC Dallas), Tyler Adams (New York Red Bulls), Alejandro Bedoya (Philadelphia Union), Lynden Gooch (Sunderland), Weston McKennie (Schalke), Kelyn Rowe (New England Revolution), Danny Williams (Huddersfield Town); Juan Agudelo (New England Revolution), Dom Dwyer (Orlando City), C.J. Sapong (Philadelphia Union), Josh Sargent (St Louis Scott Gallagher).