Presumably spent too long at McLaren working under Ron.
The statement doesn't specifically say he'll be ready for the start of the season, so they're leaving that as a bit of a question mark for now.
It was already going to be a tough ask with only three days of testing an hour less running each weekend and limited to grand total of 126 laps across three months in the 2020 car. If he misses pre-season after two years out it will be a disaster.
He's not had much luck at the start of the year since he moved to McLaren, what with missing a race in 2015 after his testing crash, then missing another race after his Melbourne crash in 2016.