When he came to the major leagues he would shake off catchers calling for fastballs to throw his splitter. That would get him in trouble because he would fall behind in counts and either walk hitters or be forced to throw a fastballs when hitters were expecting them.
The Yankees finally had enough and shipped Contreras to the Sox for Esteban Loaiza in a deal they likely regret.
Like the Yankees, the Sox are trying to convince Contreras to use his fastball more.
"He has so much confidence in his split-finger, he likes to throw it," Guillen said after Contreras and the Sox fell 4-2 Thursday to the Oakland A's. "It's sometimes hard when he made a living with this pitch. Now we try to see if we can change that."
The loss was only the second for Contreras since coming to the Sox in a July 31 trade. He's 4-2 with a 3.91 ERA in seven starts with the Sox.
He lasted 51/3 innings Thursday and yielded four runs on four hits, walking four.
Contreras said he didn't have a good feel for the fastball Thursday, so he went more with his splitter. But, as he has said in the past, he knows he's a power pitcher with a good fastball.
"The problem is I don't have command of the fastball yet," Contreras said with help from third base coach Joey Cora's interpretation. "Once I get that, the splitter will be more devastating."
Contreras walked Bobby Crosby leading off the third. Billy McMillon then homered down the right-field line to give the A's a 2-0 lead.
Marco Scutaro singled and, after a flyout, Contreras hit Eric Byrnes. Then he threw a wild pitch that allowed Scutaro to score from second for a 3-0 lead.
The Sox came back with two runs in their half of the thirdon sacrifice flies from Aaron Rowand and Juan Uribebut a leadoff walk hurt Contreras again in the sixth.
This time he walked Scott Hatteberg, who scored on Damian Miller's double.
"Today wasn't throwing the ball over the plate," Guillen said. "But that team will take a lot of pitches and make him throw strikes."
The Sox, as the Yankees did before, basically are asking Contreras to change a pitching pattern that made him attractive to big-league clubs in the first place.
In his native Cuba, Contreras was able to use his split-finger at will. But it has been a different story facing major-league hitters.
Guillen said Contreras has been better about his pitch selection in every outing for the Sox, and Contreras seems eager to have as much confidence in his fastball as he does with his splitter.
"That's one of the things I want to work on, getting more command of the fastball and then finishing off [hitters] with the splitter," he said.