This trophy seems daunting, but it is actually quite easy. Start up a quick play game on Rookie difficulty. Choose for your team, the Atlanta Falcons. You can play against whoever you wish. (I recommend the Cleveland Browns since they suck)
Every offensive possession you have, make sure you choose a pass play. I threw all my passes to the tight end Tony Gonzalez. He has probably the best hands in the game and will catch literally everything you throw at him (as long as it isn't 20 feet over his head). He will most likely get fatigued at some point and that's okay. Just call a couple run-plays until he comes back on the field.
Once he has logged 21 receptions, all you have to do is complete the game and the trophy will pop!