It's interesting to note that Jake Ryan was tied for 5th. in the league among ILBs in run stop percentage in their 2017 rankings whereas Martinez does not appear on the list.
There has been a tendency in these pages to view Ryan as "just a guy" or a bum. While his coverage skills have been disappointing, anybody with a focused eye didn't need to see this PFF ranking to know he was quite good against the run last season.
Even looking at blunt instrument statistics, courtesy of Football Outsiders, ESPN.com and PFF we find:
Martinez:
Overall 2017 PFF grade: 66.6 ("below average")
https://www.profootballfocus.com/nfl/players/blake-martinez/10765
982 defensive snaps
96 solo tackle, 9.8% of snaps
48 assists, 4.9% of snaps
10 tackles for loss
1 forced fumble
8 passes defended
1 interception
Ryan:
Overall PFF grade: 75.7 ("average")
https://www.profootballfocus.com/nfl/players/jake-ryan/9562
507 defensive snaps
52 solo tackles, 10.3% of snaps
29 assists, 5.7% of snaps
3 tackles for loss
1 forced fumble
0 passes defended
0 interceptions
While the general perception is that Martinez was the tackling machine last season (with his eye on 200 this season, LOL), it's worth noting Ryan had higher tackles per snap percentages in both solos and assists.
In terms of those overall grades, Ryan would benefit as a rotational player where his snaps were more limited to doing the things he does best (269 run snaps vs. 238 pass) whereas Martinez had to play to both his strenghts and weaknesses being a 3-down guy where the signal calling responsibility finally fell, out there playing to both his strenghs and weaknesses.
The PD and INT numbers notwithstanding, I think it is fair to say Martinez straying far from the line of scrimage in coverage is not a good look for him. He may have been better in downfield coverage than Ryan but not by much, ergo drafting Jones and then Burks. I don't know how Martinez's PFF grade broke down in run vs. pass, but I'd surmise Martinez's pass defense grade was poor.
Jusdging from this limited view of one preseason game, Burks is not a complete solution. The Ryan run defense role still needs to be filled unless Burks makes some startling jump in the run game as we move through the preseason. The loss of Ryan is not extraneous.