I think Rafa sees a future star in Lascelles - hence the captaincy (leadership) and when he's on, he's really a mountain of a player. His youth and inexperience are the reasons why he can be so inconsistent with his mistakes that have been quite costly to us. He's a decent footballer and has been a threat on set pieces, etc.
Clark is playing his best football under Rafa and I think could very well do a job in the PL as he's a bit of a late bloomer, as TCD says above, he's been playing for shit managers and shit teams its been impossible for him to stand out.
If we sign a top 1st choice to partner Lascelles or Clark we'll have a fine rotation imo.