Henman knows a lot about tennis, as he was a pretty solid tennis player at his peak, but he also watches the sport a lot after his playing career. He's an analyst these days, mostly working around the Grand Slams, so he's quite familiar with how the Australian Open went.
He watched Djokovic struggle and lose to Jannik Sinner, but his opinion of the man and the tennis landscape didn't change too much. Speaking to Sky Sports, he recognized Sinner's surge and also the improvements of his fellow youngster Carlos Alcaraz in recent years, but for him, Djokovic is still the man to beat.
"If Alcaraz and Sinner stay injury free they’re going to play each other a lot in the biggest and best tournaments. To have these new rivals at such a young age with so many opportunities, that’s hugely exciting."
The Australian Open outcome will likely motivate Djokovic greatly, and he's always been very good at coming back strongly. It's also unrealistic to expect Sinner and Alcaraz to be amazing all the time because that's just too much to expect from the youngsters.
"However, for me Djokovic is still the player to beat. He won three of the four majors last year. By his own admission he didn’t play well in the semi-finals of Melbourne but he’s still the man to beat."
The rest of the field is pretty strong, but they haven't really proven they can take on Djokovic consistently, so as long as the Serbian looks remotely like himself, he'll be the man to beat.
It's far too early to call this any kind of decline. He was simply beaten by a better player on that day in Melbourne.