Stralman isn't what he was 2-3 years ago but he's still a solid 2nd pairing defenseman who takes on top lines. Girardi is obviously well past his prime but I would still take Stralman and Girardi over whoever makes the team in Witkowski, Ruuta, or Schenn and who knows if Shatty bounces back or continues to struggle. As I mentioned, they also lost JT Miller. I know he's not a great player but that's still a 50+ point player out of the lineup. I wasn't even saying that any of them were huge players for Tampa, just correcting the OP which said they didn't lose anyone.
Stralman truly is not a solid 2nd pairing guy anymore, or at least he wasn't last season. I'm hoping he makes a full recovery from his injuries and recaptures his game, but it was brutal watching him at times last year. I really like the guy so I hope it was just his injuries weighing him down, but he was NOT a positive contributor last year.
As for Witkowski, Rutta, and Schenn - they will likely be used sparingly.
The top 6 will be Hedman, McDonagh, Cernak, Sergachev, Coburn, and Shattenkirk. 22 definitely comes with his warts but I wouldn't exactly consider him a massive downgrade from Girardi.
Miller is certainly a loss - he gets overlooked, but he's a good player. That being said, he was more of a luxury than a necessity for the Lightning and spent a lot of time on the 4th line.