OLB or MLB? That is a major difference for speed and strength requirements. MLB should have very high int (17plus). The rest skills are similar for both tack, pos, vis, agi and catching would be great for interceptions.

i wouldn't worry too much about catching to start off with, no one seems to have found much of a link between LBs with high catching and those making the interceptions, though might be worth training up later

though as previously said, as a starting point for LBs;
OLB
speed - 15+
str - 13+
and look for good starts in tackling, position, vision & agility

MLB
Int - 15+
Str - 15+
spd - 12+
and look for good starts in tackling, position, vision & agility