Sky F1 pundit Paul di Resta will be sitting this year on McLaren's F1 bench alongside Stoffel Vandoorne and Nyck de Vries as a third addition to the Woking-based outfit's potential reserves.
Di Resta, who last raced full-time in F1 in 2013, with Force India, and who substituted for Felipe Massa at Williams in the 2017 Hungary GP, will be on stand-by on weekends when calendar clashes between F1 and FE will preclude Vandoorne and de Vries from embedding with McLaren.
"Our main focus is on this agreement that we are having with with Mercedes," said McLaren team boss Andreas Seidl.
"As you know also with what we would have had in place last year in Silverstone, we have a good relationship also with Paul di Resta, who is also in possession of a super licence.
"So we have an agreement with him as well in order to jump in if the requirement is there. So we're prepared for different scenarios."
Vandoorne has been assigned to McLaren for this weekend's Emilia Romagna GP, although the Belgian is not currently present at Imola.
F1 super-sub Nico Hulkenberg, who replaced Racing Point's Sergio Perez and Lance Stroll at three events last year, will act as Aston Martin's official reserve this season but the German could also be considered by McLaren, although the team will rely on Mercedes to make the final call.
"Again, in the end, we are depending on Mercedes," added Seidl. "We simply have to follow what Mercedes is putting in place for the different weekends. But our main focus is clearly on Stoffel and Nyck."