Grimsby set up the tie by stunning United at Blundell Park in the previous round.
Artell said: “Everyone agrees that Manchester United are still finding their way under a new manager, who needs to be given more time.
“You can see the fruits of the green shoots of progress from their side — but Brentford are not that team. Brentford are a well- established team.
“They’ve appointed from within to continue the work the previous manager did — and everyone would suggest that Keith Andrews is doing a brilliant job and he’s taking them on.
“You don’t beat West Ham so comprehensively, you don’t beat the reigning champions, if you haven’t got those principles of play already ingrained. I’m not sure that was the case with United.
“I don’t want loads of letters from United fans from Kenya or Norway — or Stretford — but they’re still in that process of finding their way again.
“Brentford are fully in, fully understanding. That makes it so much more difficult.”