I think Son is reasonable cover for Lamela. However last year we do seem to have players in the squad who could give us the same as what Alli and (especially) Eriksen did.
Conversely to some others on here, I wouldn't mind seeing Mason given a go as Alli's understudy. He is decent on the ball, prepared to work and fight and play a part in the press and I think he is also capable of making those same runs in behind the opposition's defence like Alli does (as Mason did against Sunderland early last season). The number 10 role was also the one that Mason played throughout his many years in our youth teams. Whether or not Wjinaldum is better than Mason here I have no idea, but if given games at number 10 I think Mason would both score and create goals for us. I would say that he is a good enough player to offer cover for Alli and rather than spending probably 15 to 25 million on a player like Wjinaldum. IMO we should instead look to concentrate our remaining efforts (and funds) on a player that provides competition/cover for Eriksen. I think this will prove to be far more difficult however. We would need a player who is creative and skillful enough to play as a number 10, but is also prepared to work hard enough to never shirk his responsibility in tracking the forward runs of the opponents right back when the opposition turn over the ball. There are not too many skilfull players out there that have that sort of desire and work rate.