Walcott, who turned 28 on Thursday, was a surprise omission from Gareth Southgate's squad to face Germany and Lithuania, with the likes of Nathan Redmond and Jesse Lingard preferred instead.
The former Southampton youngster has enjoyed one of his best seasons to date and has hit 17 goals in just 24 appearances across all competitions.
"He has become more of a complete player, more efficient," the Frenchman said, when asked about Walcott's development.
"He does better in the tactical part of his game and works hard defensively and offensively. He has been less injured as well, that helps.
"He has turned up in big games as well, he can score goals in big games. He showed that against Bayern, he scored the goal and I think it was a penalty on him and he can influence games.
"The major quality of Theo is the quality of his receptions - that means he gets the ball in areas where he is extremely dangerous and that is a very intelligent part of a football player."