No offense but it's in competitions like these that we tend to see the best of Man UTD. Grimsby may be a good team in their division but I doubt they would be able to withstand the pressure of a top side like the Red Devils. I can't predict the correct score but I know that Man UTD would win by a huge margin.
Aren't we always pay attention with Manchester United in any competition? 
Grimsby Town isn't even a good team at all, they're mediocre in League Two. The gap level between Manchester United and Grimsby Town is big, even though Manchester United didn't win the last two match, but their performance weren't awful. First match they were treated unfair, the second match Bruno unlucky to miss penalty.
Manchester United should able to win with two goals difference.