I've never seen Seager play, but I'd like to.
I'd never seen Gallager play until I went to the Yeovil match last year. He played well and scored. I can confidently say that I don't believe Koeman would have played him.
Reed is worth a game a month IMO, and if he isn't ready then neither is Clasie.
Interestingly, JWP plays regularly and I see nothing of merit in his performances (this may make me a hypocrite), I can't believe Reed would be any less effective.
Also Targett was awful when he first started in the team, but after a run of games started improving, confidence is fragile but makes all the difference.