The Spanish double world champion was named as a McLaren Racing ambassador during pre-season testing, with the team confirming then that he would drive at selected F1 tests during the year.
Next week's test will be based around evaluating the latest Pirelli tyre compounds and Alonso's track time will be dictated by a run plan designed by the Italian tyre supplier.
"The job is not finished though, I want my first points of the season, so now it's time to focus on the race", said the Spaniard.
"As I said a year ago, and in Barcelona [testing] as well, my idea is not to come back", he told Motorsport.com.
"I had great opportunities outside F1 to do something unprecedented so that's why I took the decision".More news: Changes coming at FAA after crashes, watchdog says
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Since departing Formula 1 full-time Alonso has continued preparations for the Indianapolis 500, raced in the World Endurance Championship, and also tested Toyota's Hilux that it competes with at Dakar. Coming back is not in the plan.
The team's other race driver, Lando Norris, will share Tuesday with Sainz and get a whole day in the auto on Wednesday.
They believe they have solved the issues for 2019 but Alonso will potentially give them a clearer view of progress than Sainz and Norris, who have very limited experience of last year's 2018 vehicle.
"Honestly, I'm not missing too much at the moment as I've been very busy". I think the team decided this was the best moment and not taking time from the race drivers to develop the vehicle. "I don't close doors 100 percent to anything in the future". "It might be different if I was sitting at home on the sofa".
Alonso will drive all day on Tuesday, and share the duties with Sainz on Wednesday, running a second McLaren to test Pirelli tyres.