While the Rangers remain on the lookout for options that may improve their 2013 starting rotation, it appears the core of the rotation will all be locked up through at least 2017.

Three sources acknowledged Wednesday the club had agreed to a five-year, $55 million contract extension with left-hander Matt Harrison. The deal is expected to be announced Thursday, a source said. The deal will also include a club option for 2018 that can also be vested.

"We're still talking," Harrison said at Rangers FanFest last weekend. "We're excited about the possibility of a long-term deal, but we still have thinking and talking to do."

Harrison, 27, is in his second year of arbitration eligibility and was scheduled to exchange arbitration figures with the team on Friday. The deal buys out three years' worth of free agency.

In addition, the Rangers were one of four teams on hand to watch right-hander Javier Vazquez throw five shutout innings in a Puerto Rico winter league playoff game. Vazquez, 36, is weighing a return to the majors after sitting out 2011. The Rangers also maintain contact with agent Scott Boras about right-hander Kyle Lohse.

The team hopes in the future, however, to not have to chase free-agent starters all winter, and the Harrison extension will move them closer to the goal. Right-hander Yu Darvish, 26, is signed through 2017 (though Darvish could take free agency following 2016 if he finishes in the top two in the AL Cy Young voting twice in the next four years). Left-hander Derek Holland, 26, is signed through 2016 with club-friendly options for 21017-18.

Holland earned his contract, which could be worth up to $45 million, after a strong second half in 2011 that culminated with a dominating World Series performance. Harrison, however, has been more effective overall the last two years. He pitched a career-high 213 innings last season — Holland has yet to eclipse 200 innings — and tied the club record for wins in a season by a lefty with 18.