This is one of the reasons that ERA is a really poor stat for evaluating a pitcher. It's useful when taken into context (which is what stats like FIP and xFIP try to provide), but asking for what I would consider an acceptable ERA for 2013 is like asking what I think an acceptable number of wins would be. Too many variables are at play for it to be predictive and it often doesn't do a very good job of telling you how good a pitcher actually was in any given season.
This is a different issue, and one that is unlikely to get resolved as I think ERA is the single simplest and most useful measure of a pitcher's effectiveness. A pitcher's job is to not allow runs-regardless of where they are pitching. There will be a slight variation in different ballparks, but good pitchers pitch well anywhere. They prevent the other guys from squaring up; they strike out batters; they do not walk batters. There is no need to use other metrics as the primary tool to evaluate pitchers when a simple very revealing statistic already exists. This is not to say that there is no place for the other metrics. I quoted, for example, WHIP, in one of my posts. Thats a useful statistic because in the days of pitch counts you need to be efficient so you can go deep into games and save the bullpen, which makes you a more valuable pitcher. FIP, not so much. It purports to be defence independent, but its not really independent at all. See this very good article:
Fielding Independent Pitching (FIP) Isn't Fielding Independent
To understand how far from reality the names of some statistics are look no further than FIP. This is an acronym for Fielding Independent Pitching.
According to Wikipedia the formula for this statistic is as follows:
FIP=13HR+3BB-2K/IP
You will notice IP (innings pitched) is the denominator. The formula for a pitcher's IP is the number of outs made while he was pitching divided by 3.
Of course "outs" are hardly fielding independent. Even a pitcher who strikes out one batter per inning has fielders who help get the other two.
So, in fact, this statistic is not fielding independent at all, despite the label its creators put on it. Do the results of a pitchers fielders actually change the pitcher's FIP? Lets look at two scenarios where a pitcher faces 9 batters:
Scenario one:
groundball hit, groundball hit, groundball hit,
strikeout, walk, walk, strikeout, home run, strikeout.
In this scenario his FIP is 13+6-6/1 for 13.0 FIP
Scenario 2:
groundball out, groundball out, groundball out,
strikeout, walk, walk, strikeout, home run, strikeout.
In this scenario his FIP is 13+6-6/2 for a 6.5 FIP
If the fielders catch those groundballs and turn them into outs, they cut the pitcher's FIP in half.
Is FIP really "fielding independent"? Not hardly. But you wouldn't know it from its name or how it is used around the internet.
Not a big fan of that particular metric.
And another article recognizes that ERA is the gold standard when it comes to measuring a pitcher's effectiveness when it concludes:
xFIP has the highest correlation with future ERA of all the pitching metrics
If you need something to correlate with ERA, its better just to use ERA instead. And if you want to try to predict future ERA I would use the most accurate metric, SIERA, not xFIP. I am also not a big fan of SIERA, although I have a rough understanding of what it attempts to do. Lackey had a SIERA of 4.22 in 2010, and thats classified as "below average". I would submit that his ERA since 2007 indicates decline, and even without an elbow injury he would likely have continued that decline, further raising his SIERA and ERA.
You can value xFIP more than ERA, but I don't. Old folks like me just like to keep things simple when there is no need to obfuscate the data.