The Boston Red Sox are left to wonder what could have been.

Jon Lester signed a six-year, $155 million contract with the Chicago Cubs last week. The Red Sox were the runners-up in the Lester free agency sweepstakes, but it’s possible Boston could have signed the left-hander to a contract extension last spring if it offered him something in the range of five years and $120 million.

“That is one of those deals where hindsight is 20/20. You go back in time and you look at it and you go, ‘probably yes,’ ” Lester said Thursday on WEEI’s “Hot Stove Show.” “I mean, you don’t know. I mean, it is one of those deals where when it is sitting in front of you, that is a lot of money to turn down. That would have made it very difficult to turn it down.”

Philadelphia Phillies ace Cliff Lee signed a five-year, $120 million contract as a free agent in 2010 — hence why the figure was used as a talking point during Thursday’s radio appearance — and such an offer, in hindsight, would have been a better starting point than the four-year, $70 million contract the Red Sox reportedly offered Lester during spring training. Lester said he wasn’t insulted by the initial offer, though. After all, it’s a business.

“I think that’s an easy topic to pick on for a lot of people,” Lester said. “We’re all men. We all understand what they were trying to do. By no means was I insulted. By no means were my feelings hurt. They had a game plan and we had a game plan, and we went into it with our plans. Just, for whatever reason, we couldn’t get anything done before the (2014) season started.”

Red Sox general manager Ben Cherington said last week at the Major League Baseball winter meetings in San Diego that he wished the sides had more dialogue about an extension before Boston traded Lester to the Oakland Athletics in exchange for outfielder Yoenis Cespedes at the non-waiver trade deadline. Lester said Thursday he thought there was an agreement from both sides they wouldn’t negotiate during the season.

“Like I said the other day, I don’t know if that is a bad quality or a good quality, but I am kind of hard-headed when it comes to that,” Lester said Thursday of the decision not to negotiate during the season. “If we make a decision one way or the other — just like if we would have made the decision to continue talking — I would have expected that to continue. I think we all kind of decided at that time, with the distractions of everything going on, it wasn’t the right time or place to continue the discussions.”

The Red Sox’s final offer reportedly was for six years and $135 million — far greater than their initial four-year, $70 million offer, yet still short of the Cubs’ six-year, $155 million proposal with a seventh-year option.

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