Itchy and Scratchy will learn tons from Pep, the reality is he has the raw material, he's young and was prepared to work his arse off and eventually force Pelle to give him Bony's spot despite the manager being desperate not to do so.
Pep's going to have to do a lot of work on Kelechi's weaknesses anyway with players he has to make succeed (ie. Sterling) so Kelechi will benefit from that, as well as from the constant rondos and the general football dominated intense training sessions Pep demands day in, day out, so Kelechi will only get better from the sheer amount of practice he'll be asked to put in.
Will he be able to exclusively cover for Kun? I doubt it so early on, but with the Sane's and Sterling's around the squad, there shouldn't be THAT much onus on him. Plus with the Brazilian kid coming in in the winter, added to Nolito, there'll be added goals in the squad anyway.
I don't see us signing another striker this summer and see Kelechi as #2, maybe even #1 in the first few games unless Kun makes huge strides in the next fortnight.