ST. LOUIS -- With Shane Robinson and Matt Carpenter having outperformed him in April, Erik Komatsu learned on Tuesday that his time in the organization has likely come to an end.

The Cardinals designated Komatsu for assignment in order to open roster space for Allen Craig, who came off the disabled list. Though most players simply have to clear waivers when designated for assignment, Komatsu's situation differs because of his status as a Rule 5 pick.

Komatsu will be placed on waivers first, giving any other club the option of taking him. The team that does will have to abide by the rule requiring Komatsu to remain on the Major League roster all season in order to stick with that organization.

If Komatsu clears waivers, the Cardinals will have to offer him back to the Nationals for half of the $50,000 they paid to select him from Washington in December's Rule 5 Draft.

With the obstacles standing in the way of retaining the 24-year-old outfielder and sending him to their Minor League system, the Cardinals anticipate losing Komatsu.

"He's going to get snatched up by somebody," manager Mike Matheny said. "From what I've seen, I'd like to have the kid around. I'm sure somebody else has seen the same thing."

Komatsu earned a spot on the Opening Day roster after impressing the staff during his first big league Spring Training. He recorded his first Major League hit in his first at-bat but had only three in 20 at-bats since.

With Craig able to play the outfield, the Cardinals did not need to keep Komatsu as another backup. The club also favored keeping a right-handed-hitting outfielder (Robinson) over the left-handed-hitting Komatsu in order to balance the bench.

"With what Shane has done and with what Matt has done, that really put us in a tight spot," Matheny said. "It came down to numbers. That was a tough conversation to have with [Komatsu]. We're very excited about what he did."

Craig off the disabled list and in the lineup

ST. LOUIS -- Following a seven-game rehab assignment, Allen Craig was activated from the disabled list on Tuesday and immediately plugged into the lineup to play first base and hit seventh in the series opener against Pittsburgh.

In order to make room for Craig on the 25-man roster, the club designated Rule 5 outfielder Erik Komatsu for assignment.

Craig wrapped up his rehab stint with a three-hit, two-homer, five-RBI performance as Triple-A Memphis' first baseman on Monday. In his seven Minor League games -- three at high A and four in Triple-A -- he went 10-for-28. He played first base in each of his final two appearances.

"I've been feeling good in the box for a couple of nights," Craig said. "Last night was a good way to end my rehab assignment. I felt good about it."

With Lance Berkman on the DL, Craig is expected to get frequent starts at first base. He will not, however, play every day right away, despite his insistence that he feels capable of doing so.

Manager Mike Matheny will keep Matt Carpenter in the mix so as not to overwork Craig, who played no more than two consecutive days during his rehab assignment.

Craig also discarded any notion that his timetable was set back by a late March push to get him ready for Opening Day. Though he took an extended break from game play after the soreness in his surgically repaired right knee persisted, he is coming back as initially scheduled. When he underwent the surgery in November, the Cardinals projected that he'd come off the DL for the second month of the season.

"I lost a lot of strength when I had surgery, and ... [it] was a lot of hard work to get the leg strong," Craig said. "I'm in a good spot now and feel good about it. I have to be on top of maintaining that strength. Other than that, the knee feels great, and I'm not worried about it at all."

Though Craig can also play the corner outfield spots, he likely won't see any playing time there initially. With a healthy outfield and the opening at first base, the Cardinals have no need to push him more than is necessary.

"From the reports we're getting from Memphis, it looks like he's moving well and feels good," Matheny said. "He responded well to playing the field."

Home-plate umpire Bucknor leaves game early

ST. LOUIS -- The Cardinals and Pirates finished their series opener on Tuesday with an umpiring crew of three after CB Bucknor exited with an undisclosed injury in the seventh inning.

Bucknor, who was assigned to home plate, was in obvious discomfort, stepping away a few times in the seventh. At one point the Cardinals' trainer came out to visit with Bucknor, who was joined by the rest of the umpiring crew during the discussion.

A few minutes later, Bucknor asked second-base umpire Dan Iassogna to return to the dressing room, where Iassogna put on his plate gear.

When the Pirates made a pitching change during the bottom half of the inning, Iassogna took Bucknor's place behind the plate. Bucknor did not return.

Berkman not optimistic about Friday return

ST. LOUIS -- Though Lance Berkman said it remains a possibility that he'll be ready to return from the disabled list on Friday, he did not sound overly optimistic about such a timetable.

Friday is the first day that the veteran first baseman, who is recovering from a strained left calf, would be eligible to come off the DL. That day also coincides with the team's first trip to Houston, where Berkman spent the first 12 years of his Major League career.

"It's just one of those things, when you're dealing with a muscle injury -- it can make rapid improvement and then hit a plateau," Berkman said. "You don't know how long it's going to take to get it back to 100 percent. Right now I'd say it's probably 85 percent. But before I'm going to go back out there, I'm going to get it as close to 100 percent as possible, because what I don't want to have happen is go out there too soon and end up right back here again. The whole point of this DL stint is to get this totally behind us."

Berkman did resume running on Monday, though it was not a strenuous test of his leg, and he continues to hit in the batting cages. It is unknown whether Berkman would need to go on a Minor League rehab assignment before being placed back on the 25-man roster.

"We'll see how the next few days play out," he said. "I've got to run. I've got to really push it. I've got to go full speed and have it not feel bad the next day. That's what the next step is."

Worth noting

• The Cardinals wrapped up April leading the National League in run differential with a plus-53 mark. Only the Rangers have a better mark, with their plus-56 margin. The Cardinals' starters posted the second-best ERA (2.57) in the Majors last month, and the pitching staff also ranked second overall, at 2.61.

"[I'm] very proud of how they've played the game and how they showed up every day," manager Mike Matheny said. "They had some guys not feeling 100 percent but pushing through. Overall, you can't be any more pleased with the pitching than we are. Looking back, it was a good month."

• Hall of Famer Stan Musial will soon have a bridge named after him in his home state, Pennsylvania. A state bill is expected to be passed later this week that will rename the Donora-Monessen Bridge over the Monongahela River after Musial, who is from Donora.

• Top pitching prospect Shelby Miller earned his third straight win in Memphis' 5-4 victory on Monday. Allen Craig, who rejoined the Cardinals on Tuesday, drove home all five of Memphis' runs. Miller allowed four runs on seven hits and struck out four in five innings.