I don't believe in lineup protection. But there is something similar that I haven't yet been disabused of. I believe in lineup depth. It's the idea that a lineup with real threats 1-9 is more difficult to pitch against than a few great hitters and a few automatic outs.
To look at lineup depth, I created hypothetical best lineups for all the AL teams, and ranked hitters not by position or where they bat in the lineup, but by their wRC+ this season. This is a comparison of one team's best hitter against another team's best hitter, worst hitter against worst hitter. Evan Longoria's return might not make Matt Joyce and Ben Zobrist any better, but it does push them into a comparison against other second and third best hitters, so it makes them seem better, and it makes the Rays' lineup deeper.
Here's how it looks without Longoria (click to expand):
That's pretty mediocre. The bottom of our lineup is very good, but the top will not get it done. Matt Joyce is a wonderful hitter, but when he's the best hitter on your team, you're in trouble.
And here's how it looks with Longoria (click to expand):
This is much better. As a number one hitter, Longoria is average. But his presence bumps Joyce down to the second tier, where he's also average. Same deal with Zobrist as a number three. Keppinger actually has been an above average fourth best hitter. Once we pass Kepp though, the rest of the lineup plays out identically, with a big drop at Jennings and a gradual run up to good relative production.
We can be glad to have Longoria back for his bat, but also for how his presence elevates the top of the Rays' lineup from pretty punch-less to perfectly respectable.