Ratings are just physical skills, doesn't mean they will perform.
I've had 3 and 4 star players start over 6 star players many times because they still performed great.

I started this season with a 3 star DMC that I've had for 10 seasons (there's a reason he's been there that long!) and he'd still be starting more than likely except i changed my formation mid season.

The funny thing about him was I didn't really want him when I got him on Day 1, but no one had bid on him with 10 seconds left so I figured eh... 1 token, why not? Lucky break there!
Other times my prized acquisition (usually a 5 star fast trainer) busts.

3 seasons ago I got an 18 yr old AMR on Day 1, he was the only one available, so I got him almost by force, and he's been a consistent 40 goal scorer. Same season, the PERFECT AML for my system appeared on the negotiation list. He also had good stats on that guy's team so I made him an offer he couldn't refuse, and he has broken my goal records 2 years in a row!

So the answer is.... be as smart as you can, avoid players that other managers obviously don't want, and go after ones that are young or have proven success from their previous squads, and hope for the best. But most importantly, when you get one that is playing well, keep him!! Do not assume the younger/higher rated player is going to automatically play better, because that just isn't going to happen too often.