NEW YORK -- Yankees GM Brian Cashman is aggressively trying to upgrade his starting staff, which has seen four-fifths of its opening day rotation end up on the disabled list.

"I have to reinforce our pitching, in my opinion," Cashman said in a phone conversation Thursday. "I have things that I feel I have to try to do, that I'm trying to do, but it is easier said than done."

Masahiro Tanaka was the latest Yankees pitcher to land on the disabled list this season. The Yankees' opening day rotation has lost four of its five pitchers to the DL. AP Photo/Ted S. Warren

Cashman is in go-for-it mode, even though the Yankees, 47-47 on the season, start the second half five games behind the Baltimore Orioles in the AL East and 3½ back of the Seattle Mariners for the final wild card.

The Yankees' starting rotation has lost Ivan Nova and CC Sabathia for the season. Michael Pineda and Masahiro Tanaka are both on the disabled list. Both could possibly return in August.

The Yankees' second-half rotation consists of Hiroki Kuroda, rookie Shane Greene, David Phelps, Brandon McCarthy and Chase Whitley.

"We have to try to improve, reinforce and upgrade, certainly," Cashman said. "We certainly we would love to have some significant upgrades but when you lose four out of five starters, it is hard to re-materialize the same type of abilities with the guys you lost. It is whether you incrementally upgrade."

The Yankees could use their financial might to trade for someone like the Phillies' Cliff Lee. Lee, who has a no-trade clause, is making $25 million this year. He has another $25 million coming to him in 2015, while there is a team option for $27 million for 2016 or a $12 million buyout. Lee is scheduled to come off the DL and start Monday for the Phillies. He could make two more starts prior to the July 31 trade deadline.

Recently, Cashman called up David Huff to replace Alfredo Aceves and traded Vidal Nuno to Arizona for McCarthy.

"Whether it was Huff over Aceves or you significantly upgrade, which we feel like we are trying to do with McCarthy over Nuno," Cashman said. "Those are the attempts. Those are the efforts."

Cashman still wants to do more.