Think Lambert still needs to prove if he's up for the big stages, as in fact almost 100% of our squad need to if we get promoted, as I think there are very few who have had a decent run in the Premier in their careers so far (only remember Davis, and it wasn't a trouble-free time at Sunderland).
So far he's proven to be up for the Championship, but apart from this being his first season at this level (can't thank him enough for what he's done though), the Premier is a world away from the FL and it's not like he'll be recieving as many balls from Lallana or Chaplow as he recieves now. A striker may be good at finishing but may lack the skill or creativity to create chances for himself, which he will have to do more in the Premier as we won't be dominating play as we've done in the last 2 seasons.