How can the Mets possibly become better than they were last year, considering they will be without Matt Harvey and cannot expect another Marlon Byrd? Thinking about this logically, if they signed Shin-Soo Choo, which I don't think they will, he'll essentially put up the same numbers Byrd did last season, but at 15 times the cost. At that point, half of the payroll that came off the books is eaten up and the Mets presumably will fill the hole left by Harvey by signing a veteran starting pitcher costing approximately $10 million. Now that's basically Sandy Alderson's budget right there with a little wiggle room left and the team is no better off than they were last season, barring some epic comeback by Ike Davis or a breakout season by Lucas Duda, Josh Satin, Travis d' Arnaud, etc. It just doesn't seem to add up to me.
You're right, the replacement for right field will not be a major upgrade over Byrd, though it's worth pointing out that Choo did produce more than Byrd did last year. It's safe to say that spot will be a wash or a small step down. However, left field was a big weak spot for the team and whoever steps in for Eric Young Jr. is very likely to provide superior offense.
Perhaps the biggest boost will come from the shortstop position, where the Quintanilla/Tejada tandem can best be described as a black hole in the lineup. A competent batter in that spot, allowing Juan Lagares to drop down to eighth in the order, will completely change the structure of the lineup and give it some much-needed depth.
Catcher will also see an upgrade as even an average showing from d'Arnaud should trump what Buck provided. It's hard to see pitching as improving much, if at all, but for every Harvey start in 2013 there was a Aaron Laffey, Aaron Harang or a Shaun Marcum start. With more pitching prospects finally approaching major league readiness, there's a lot of depth that should help minimize the number of ineffective starters the team needs to rely upon.
As far as how all of this fits into the payroll space, it's early for any of us to know exactly how much is on the table or how much these players would command. But with so many barely replacement level players getting time this year, it's not going to take superstars to see improvement.