Sceptics may well doubt that Hakkinen can contribute anything much in such a short space of time, in an unfamiliar car at the tail-end of November and for a team who have two test drivers already in addition to a confirmed 2007 line-up of Alonso and British rookie Lewis Hamilton.
They may be wrong, however.
"A driver of Mika's calibre and experience can only bring added value to our testing programme," said McLaren boss Ron Dennis, adding that there were no plans "at this stage" for him to do further tests.
Mercedes, who are hoping Hakkinen will return for another series with them in the DTM German Touring car championship, will be happy anyway to see the Finn's profile raised a couple of notches, as will team sponsor Johnnie Walker, who recently appointed Hakkinen their 'ambassador' to promote responsible drinking and safe driving.
However, Hakkinen hinted on Wednesday that there was more to it than that.
The man who came back on the radar when he turned up in Brazil for last month's season-ender, and who has since twice spent time in the McLaren F1 simulator, told reporters that the test was "absolutely serious."
Former Minardi team boss Paul Stoddart felt inclined to believe him.
"I don't think that Ron and (Mercedes motorsport head) Norbert (Haug) are the types to play games," he told Reuters.
"I think he (Hakkinen) is doing it for a reason. Don't discount him, just because he's a bit older. Imagine if Michael (Schumacher) has a couple of years out and then said he wanted to get back in the car. Nobody would laugh at that."
The fact is that Hamilton has no benchmark for feedback, Alonso remains under contract to Renault until the New Year and neither have any prior experience of Bridgestone tyres.
McLaren are venturing into new territory. They have their first rookie driver since 1995 and not since Bruce McLaren's debut in 1966 have they started a season with an all-new line-up.
Meanwhile, their Spanish reserve driver Pedro de la Rosa turns 36 in February.
Even if McLaren are the team that brought Niki Lauda out of retirement in 1982, and handed Nigel Mansell a drive in 1995 when the Briton was 41, a genuine Hakkinen comeback remains highly unlikely.
He remains in good shape after two years competing in the DTM but he was never much of a test driver. He excelled as an out-and-out racer and, since Hamilton's confirmation, there are no vacancies in that department.
It could be, however, that McLaren have in mind a mentoring job, with one double champion helping Hamilton come to terms with another. In which case, spending some time in the car would have its advantages.
Hamilton has been groomed by Dennis for the past decade but has a tough task, mentally as much as anything, in living with Alonso and two test drivers who also coveted the race seat.
Hakkinen, the driver closest to Dennis both personally and professionally in the last decade, knows what he is up against. He made his McLaren debut in 1993 alongside triple champion Ayrton Senna.
"If this test is just a one-off, then it is probably a bit of PR," concluded Stoddart. "If he (Hakkinen) comes back for more, then it's for real."
. And aren't the Mercedes DTM and Mclaren F1 team basically one big group?
