I think we have players that could actually fill out a top tier roster......if we had a defensive big and a big time scorer. Like if we all of a sudden had DeAndre Jordan, Carmelo Anthony and a top 10 2015 draft pick added to our roster, which is a huge move.
I do think though that if the rest of our team was Rondo, Sully, Bradley, KO, Smart, Young and Bass we'd be alright.
Haha, you'd be hard-pressed to find a team in the league that wouldn't contend despite adding D.J., Melo and a top ten draft pick.
This team is not "just" [anything] from a top tier club. We have a severely imbalanced roster with noticeable talent voids at key roles. Our go-to scorer is Jeff Green (though as a big Sully fan, I'm beginning to think we should look to him more for buckets), who is at best the fourth best player on a contending squad. Our rim protector and garbage man (dunks, tip-ins, finishes off P&Rs, etc.) is Tyler Zeller, who couldn't start for a contending team unless the other four starters were perennial All-Stars or close to it.
I am steadfast in my belief that this team has a terrific future, but it's time to quit thinking that another ring is within our very grasp. Things can change quickly, but that's not likely, and it's distorted to characterize our chances of contending as being "just" one piece or two pieces away when realistically that one piece is LeBron James or those two pieces are Carmelo and Marc Gasol/DeAndre Jordan/Roy Hibbert. Again, literally any team in the NBA can say that.
edit: Except maybe the Sixers.

second edit: I recognize your inclusion of "..., which is a huge move." I'm more addressing your broader line of thinking that is shared by many on this site in which it is believed that we're "only" a mysteriously defined/interpreted "player" away from contending.