The famous British team, for whom Finn Kovalainen drove alongside Lewis Hamilton in 2008 and 2009 before losing his seat, recently made the surprise decision to oust Sergio Perez in favour of the 21-year-old Magnussen.
"I'm still surprised by the team's decision," Mexican Perez, who it is believed will switch to Force India for 2014, told Brazil's Totalrace this week.
"I didn't know the news was coming and, in fact, my contract was already signed. So I was surprised.
"Time will tell if it was the right decision," added Perez, referring to McLaren's signing of the new Formula Renault 3.5 champion Magnussen over him.
Finn Kovalainen, who is also hoping to find a race seat for 2014, told the Finnish newspaper Turun Sanomat he also cannot be sure that McLaren's gamble on Magnussen will pay off.
"There are two possibilities: Kevin will do well, or he will do badly," said the 32-year-old, who despite his experience struggled late in 2013 when he replaced Kimi Raikkonen in the competitive Lotus cockpit.
"It is difficult to say which one of the possibilities is the most likely," added Kovalainen, referring again to Magnussen's chances.
"McLaren seems to really believe in him, but I don't really know him, other than knowing that he is fast and a good driver," he continued.
McLaren is hoping Magnussen can emulate the feats of its impressive 2007 rookie Lewis Hamilton, but Kovalainen warned that the team's most recent experiment with a new driver - Perez - appeared not to pay off.
"Hamilton was able to fight for the podium from the first race, but in the past season Sergio Perez has not met expectations," he explained.
"They were expecting they could polish the diamond, but it didn't turn out like that.
"In any case, McLaren is a very strong team with excellent facilities and vast resources, so at least in theory they can create the conditions to solve all of a driver's problems.
"There is no guarantee that Magnussen will succeed, but at least he will have a great chance," added Kovalainen.