The Boston Red Sox began the season thinking pitching depth would be their biggest asset.

In some ways, it is. Nearly every member of the bullpen has met or exceeded expectations. On the other hand, the team's starting pitchers have been a disappointment one month into the season.

Baseball is a game of statistics, and the numbers clearly show the Red Sox rotation has not done the job. After the weekend, Boston's starters had a 5.75 ERA, the second-worst mark in baseball. They had been knocked out of games far too early, far too often through the first 25 games, forcing the bullpen to work 83 innings, fourth most in the American League.

Quality starts are defined as six innings or longer, giving up three or fewer earned runs. It's a tool to gauge the effectiveness of starters, but it's certainly not the measure of greatness. After all, Brad Penny's six-inning, three-run performance Sunday at Tropicana Field gave him an ERA of 4.50 for the day.

The turn through the rotation last week prior to Penny's start Sunday looked like this:

• Saturday: Tim Wakefield, 5-plus innings, five earned runs.

• Friday: Justin Masterson: 6 innings, six earned runs.

• Thursday: Josh Beckett, 4 2/3 innings, seven earned runs.

• Wednesday: Jon Lester, 6 innings, five earned runs.

• Tuesday: Penny, 2 2/3 innings, four earned runs.

That was a total of 24 1/3 innings by the starters over the five-day stretch, which means, since one game went 10 innings, the Red Sox bullpen had to work 212/3 innings, an average of more than four innings per game.

That's too much for any bullpen. Yet somehow Boston's relievers have combined for an ERA of 2.93, second best in the major leagues. Clearly, relief pitching has been the most important aspect of this team's early success.

That can't last if the bullpen is being forced to pitch half of the game each night.

Getting up and getting ready to come into games in the fourth and fifth innings every game takes its toll. If we aren't seeing the effects of that workload now, we undoubtedly will see it before too long.

Penny's start Sunday was an encouraging sign. It was his best performance this season. He appeared to be headed for disaster in the first inning after loading the bases with one out, but gave up just one run and then allowed four hits the rest of the way. It was the best his fastball has looked this season.

Somehow, the Red Sox have been able to stay near the top of the division even though David Ortiz experienced his first homerless month when healthy since joining the team. Big Papi's struggles have become a daily fascination with Red Sox fans.

The bigger concern is the team's struggles on the mound and in the field. Boston has the fourth-worst fielding percentage in the AL.

Combined with the ineffectiveness of the rotation, it's a recipe for disaster.

The Red Sox have to start pitching further into games and fielding the ball better if they hope to return to the playoffs for the sixth time in seven years.

Tom Caron is the studio host for Red Sox broadcasts on the New England Sports Network. His column appears in the Press Herald on Tuesdays.