No one should be guaranteed a spot in the team. Not even Son. With Ndombele now injured, it'd probably be a good chance as any to get back in the team though. I'd personally always start him over Lucas as he's a much better player - but I'm not sure how you can drop Lucas right now. It's exactly why I didn't cry when both Eriksen and Vertonghen were dropped for the opening game.
It's weird that people cry when Kane comes straight back into the team when we're doing well without him now all of a sudden Son is available and he should be guaranteed a spot instantly.
He hasn't played a competitive game of football since the 1st of June. It's probably not smart to throw in someone who isn't match fit against a team willing to sit back.
There is a massive difference between the 2 cases though.
When Kane has been out, it's because he has been injured, and he takes time to get up to speed.. so I and many others believe he shouldn't return straight into the starting XI. The same should apply for ALL the players.
However, in this case Son has been out due to suspension. And Mauricio has stated that he was looking sharp in training... so I don't see why he shouldn't go straight into the team. Just like Kane would if he was suspended.