In his two bullpen sessions thus far, Colon has thrown only fastballs and changeups. Pitching coach John Farrell said that Colon would begin to incorporate breaking pitches when he throws his next bullpen session on Friday morning.
Farrell noted that the velocity on Colon's pitches on Wednesday was better than in his previous session.
"I think there were a few pitches that probably had a little bit extra life through the zone than the other day, and I think those were all encouraging signs," Farrell said. "I think it's, again, too premature to put any kind of radar gun. [But] just in terms of a measuring stick for him, once he gets into games, I think there will probably be a little added adrenaline. That's totally natural. But in due time, we'll get a read on that."
With Curt Schilling out indefinitely with a shoulder injury, the No. 5 spot in the rotation remains open. Colon, who turns 35 in May, is expected to be in consideration for that slot, along with Clay Buchholz, Julian Tavarez and Kyle Snyder.
"I think as an organization, we think it's very competitive," Farrell said of the contention for the fifth slot. "Pitching is a game of attrition, and you can never have enough options for our starting rotation or the depth that might provide. ... I don't want to say [he has] time to make up, but he'll emerge into that group I think in the short run."
Prior to reporting to camp, Colon last pitched on Feb. 6 in the Caribbean Series.
"We had expectations based on the way he pitched in the Caribbean Series," Farrell said. "We had scouting reports. We saw him both on video and we saw him live. I think he's showing a little bit more arm strength than those reports indicated even at that time. So again, I think the expectation was there. But I think initially, and again this is very premature, he's on the positive side of those expectations."
For the rotund Colon, who is listed at 5-foot-11 and 245 pounds, conditioning will always be a question mark. Farrell, however, said he is pleased with the work Colon has done in camp.
"Bartolo's work ethic has been outstanding since the day he reported here," Farrell said. "His conditioning and cardio work with [strength and conditioning coach] Dave Page has been twice a day. He reports very early to get some of that work done. Then his fundamental work takes over. Then he's in with the regular group during pregame work. Then he's back in after we come off the field. So he's doing everything we're asking, and he's putting forth an outstanding effort."
Colon is scheduled to throw another bullpen session on Friday, take two days off, throw batting practice Monday and then go into a starter's mode, which would make him game ready for March 15 -- if the pitcher holds to that schedule. Colon said previously that he hoped to pitch in a game on Sunday.
"While I feel ready to go, I actually believe in what John is telling me," Colon said through interpreter Eddie Romero Jr., the team's coordinator of Latin American operations. "If John says it's the 15th, then that's the day I'll be ready to go.
"I feel very good, and after what all the trainers have been doing with me here, I've seen a lot of progress. That's very important."
Colon, who said his next milestone will be to add his slider into his throwing sessions, considers himself a different pitcher now than he was several years ago.
"Three or four years ago, I was just a thrower," Colon said. "As of now, I consider myself a lot more of a pitcher. It was tough for me, because I would just try to throw the ball past everybody. I was mainly a fastball pitcher. Now I've added a couple of pitches to my repertoire, and it's made me an overall better pitcher.
"I'm very anxious [to face live hitting], because that's when you can really tell how far along you are. That's what everybody really wants to do, because your focus is that much better when you're facing live hitters."
Maureen Mullen is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.