As reported by The National Football Post:
The New England Patriots have agreed to terms with veteran free agent defensive tackle Louis Leonard, according to a league source with knowledge of the situation. ... The 6-foot-4, 325-pounder has 33 career tackles and one sack. He has two tackles in two games this year with the Carolina Panthers. ... Leonard, 26, signed with the San Diego Chargers as an undrafted free agent out of Fresno State in 2007 ... [where] he recorded 68 tackles and 6 1/2 sacks.
Signing another journeyman DT, with Pryor, Wright and Brace all banged up may be an indicator that one or more of them is done for the duration. Keep an eye on the IR list to see if anyone pops up there. All three of those guys have been quality contributors this year and it would be a disappointment to lose them heading down the stretch and into the playoffs.
The better news could be this is an indication that McCourty's injury is in fact not serious. With the Patriots already perilously thin at Corner, it would seem unlikely for them to pick up anyone other than a DB unless Devin is A-OK.
UPDATE: After surfing about this evening for a bit, I've found some more info on Mr. Leonard. The kids over at Cat Scratch Reader looked into him pretty closely when he came on board. He's been under the tutelage of several coaches with ties to the Patriots: Romeo Crennel, Mangini, Brian Cox. He's struggled with some injuries that have impacted his playing time, and has never seemed able to catch on where he's been. To be fair, a marginally productive player is certainly one of the items on the chopping block when a new regime takes over, so I wouldn't read too much into Mangini letting him go.
He also got into a spat with Cox in training camp, but then who hasn't?
His most active season did come with Cleveland, in 2008, when he amassed 25 tackles (19 solo) in 16 games, with four starts. Since then, it's been down hill. After being acquired by the Panthers, he broke his ankle almost immediately. Even so, Carolina must have seen something in him, as they gave him a 1.6 million deal last spring (not that the Panthers have been the sharpest personnel managers lately). He struggled with a sore knee in training camp this year, had an elbow injury that kept him out of the game with Tampa, and was unceremoniously released at the end of September.
Apparently, he's big but not quick after his first step, making him a potentially decent space-taker but not much in the way of a pass rusher (1 sack in four years). He can't seem to stay healthy. He's been average at best. My feeling is that Pryor or Brace (or both) will be missing some significant time and the staff felt the need for another wide body on early downs. My snap judgment is that he's a place-holder/insurance policy.