Although the team's original intention had been to give GP2 driver Vandoorne his maiden F1 run and to use test driver Oliver Turvey on the other day of the first of 2014's post-race tests, AUTOSPORT understands that McLaren decided it was important to have driver continuity across the Bahrain and Chinese Grands Prix and the test.
Jenson Button was also offered the opportunity to run, but was content to leave both days in Magnussen's hands.
The test commenced on Tuesday morning. Sauber is giving development driver Sergey Sirotkin a day of running so he can get his Superlicence, while Williams and Caterham are also using test drivers in the form of Felipe Nasr and Robin Frijns respectively.
Caterham is carrying out its day of Pirelli tyre testing on day one, with McLaren and Williams then fulfilling that role on Wednesday.
All 11 teams are required to dedicate a day of in-season testing to Pirelli work during the course of 2014.
The Bahrain test is the first of four scheduled for this year as the testing ban is relaxed amid this season's comprehensive technical rule changes. Further sessions are scheduled for after the Spanish, British and Abu Dhabi Grands Prix.
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