Just figured a contract like that was for a center piece. I didn't look at the other guys.
I think you should familiarize yourself with the "average defensive ratings" page.
Position followed by range/glove/arm strength/arm accuracy/pitch calling:
So if you look at RF, for example, Garko's arm is fairly weak for a RF. He'd be above average in LF, but I don't like that weak of an arm in RF. He is a solid hitter.
Poole, is below average in CF too. His bat is good enough, although I prefer a higher Eye rating for my Power hitters.
Obviously, their age and bat makes them long term useful pieces. With the four Type As and Poole and Garko, you have a reasonable shot at competing. Problem with thinking you'll trade Manuel "down the line" is he is 31 and signed for 5 years and he isn't cheap.
I admire the effort you made at fielding a competitive team, and you may be able to get a WC, who knows.
If it was getting near the trade deadline and it was pretty clear that you weren't going to get a spot you MAY be able to get something worth trading him for, but each season that passes will make him less and less valuable to other owners, as he will begin to decline more and more as he ages. He should be a solid contributor for the duration of his contract though, and you definitely have a good chance of filling in the missing pieces before his contract expires, so unless someone made a great offer for him I wouldn't even consider trading him.
Additionally if ever you are contemplating trading him it is very important to post that fact in world chat. There is no better way to get the best deal that you can for a great player than to announce to the whole world that you are accepting offers for him. When you do that inevitably, you will get several offers, and inevitably half of them will be down right insulting, but you should get a few very strong offers also. I also like to make mental notes of which owners offered fair deals, and which ones attempted to rob me.
Sometimes if an owner thinks he is set for a title run and would like one more impact bat he will trade multiple good prospects to acquire that "final piece".
That is the kind of deal I would be looking for, otherwise I'd keep him around as long as I could.