It's completely understandable for you to see that it's totally realistic for that to happen. i mean, after all, like you said, it's only 2-3 catches a game right? However, it just doesn't happen. The things that make a guy miss those 2-3 completions are extremely hard to fix once they're in the pros. When we drafted him, I hated the pick because of it and did a ton of research comparing college completion percentages to pro career numbers. For franchise-type guys or even guys like Dalton and Tannehill etc they remain shockingly close their college numbers on average and RARELY see a jump of more than 3%. Favre and Moon did, but their numbers were in the 40s, and there's a couple other outliers.
Overwhelmingly, though, guys don't go from mid to low 50's in college to 60's in the pros. It doesn't mean he can't or won't be successful. It just means it's highly unlikely his percentage goes above 60.