''Unless we sign a couple of guys from the Heat, we probably are going to have a size discrepancy there,'' Dolphins defensive coordinator Kevin Coyle said.
As two of the NFL's smallest players, Grimes and Finnegan are accustomed to size mismatches. But even by their standards, the Bears present a tall order with 6-4 Brandon Marshall and 6-3 Alshon Jeffery at receiver.
Through six games, the Pro Bowl duo has combined for 56 catches and seven touchdowns.
''They're big,'' Finnegan said. ''We don't have Luol Deng, so we've got to figure it out.''
Marshall and Jeffery aren't Jay Cutler's only large targets. Tight end Martellus Bennett is 6-6, and 6-2, 218-pound running back Matt Forte leads the NFL with 46 catches and enjoys running over cornerbacks.
To compound the challenge, Grimes and Finnegan are coming off their worst game of the season in Sunday's last-minute loss to Green Bay. Grimes allowed Jordy Nelson to make nine catches for 107 yards and a score, and Finnegan's poor tackling led to two touchdowns.
Finnegan declined to blame a sore neck that limited him in practice last week.
''I can't use my neck as a crutch,'' he said. ''I just didn't tackle well. I've got to shore that up.''
The Packers mounted long drives to score twice in the final 4:09 for a 27-24 victory. Finnegan said the Dolphins (2-3) can't allow the wrenching defeat to linger.
''The Chicago Bears don't give a rip what happened against the Green Bay Packers,'' he said. ''So we've got to go out there and give them our best.''
That means clamping down on their towering wideouts, even though Marshall claimed his six-inch height advantage won't be a big deal against Miami. He ranked Grimes as one of the NFL's top two cornerbacks.
''There's not a mismatch out there,'' Marshall said Wednesday. ''This is probably going to be the best secondary we face all year. Grimes is a technician. He's super-duper focused. And we all know about Cortland - he's hyper-competitive and makes plays on the ball.''
Last week's game aside, Grimes and Finnegan have played well for the most part this season. Grimes might not be performing at his Pro Bowl level of last year, but he remains the best player in a solid secondary, while Finnegan has revived his career after an injury-plagued 2013 with the St. Louis Rams.
But they have yet to face a duo like Marshall and Jeffery, who combine their size with good speed and offer two go-to options close to the goal line. The Bears rank third in the NFL with 15 red-zone touchdowns.
''In the red zone they pose a problem because Cutler can throw the ball higher,'' Finnegan said. ''You've got to play through the receiver, play through his hands.''
Coach Joe Philbin agreed there are ways his cornerbacks can and must compensate for their height disadvantage.
''We can't do anything to necessarily elevate them in the next four days from a stature standpoint,'' Philbin said. ''They are going to have to play with good fundamentals.''
The Dolphins also hope to confuse Cutler and force takeaways by mixing their coverages and applying a strong pass rush, Philbin said. He often preaches about the need for takeaways, but the Dolphins have only three interceptions this season, with one by Grimes and none by Finnegan.
''I don't think we can play one coverage,'' Philbin said. ''I don't think we're going to press every play; I don't think we're going to be off every play. We've got to mix it up. We've got to get some pressure on their quarterback and hopefully force them into a bad decision or two, get our hands on the ball. All of those good things that good defenses do, we're going to need to do.''
By some measures, the Dolphins have been stout on defense this year. They're allowing 6.1 yards per pass, third-best in the NFL. They're also allowing only 3.8 per rush, which is eighth best.
But they're 25th in points allowed per game.
''We're not consistent enough,'' Finnegan said. ''We have the playmakers. We just don't play great all the time.''
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