i have just watched the replay and focused on watching Alex (mainly because there are few people saying he has gone missing) and I think from the first half only that - he is a different type of midfielder from both Riera and Roly, he tends to be a very basic player - receive and play - not much creativity, but not many errors. Defensively is def on the left hand side of center midfield, seems to be deeper than Roly but higher than Riera. Potentially under instruction based on positioning to help Doyle out much more than Roly has been asked to help out Muscat.
In summary I think he isn't noticed very much because a lot of the good things he does are very standard and un-remarkable