Martin, who drives a part-time Sprint Cup schedule for Michael Waltrip Racing, will drive only at Martinsville on April 7 in JGR's No. 11 Sprint Cup Toyota. Hamlin is expected to need six weeks to recover from the injury, suffered in a last-lap crash last weekend at Auto Club Speedway.
Brian Vickers, who drives a full-time Nationwide Series schedule for JGR, will assume Hamlin's schedule starting April 13 at Texas Motor Speedway. Hamlin is likely to return for the May 11 race at Darlington but could be back a week earlier at Talladega.
"Obviously having to find someone to fill in for Denny is not an ideal situation to have to be in, and when you start a process like this you obviously begin to look at the drivers that are not only available but also able to drive for your race team and manufacturer. We were a bit premature in determining Mark's status past Martinsville, however," JGR president J.D. Gibbs said in a statement. "We're real happy to have the opportunity to get Brian in our Cup cars and with him driving Nationwide for us we think we have some continuity there that is beneficial.
"We really appreciate everyone at Michael Waltrip Racing and Toyota for working with us through this process. The good thing for us is that we have drivers the caliber of Mark and Brian to help fill in until Denny comes back."
Martin and Vickers split driving duties for the No. 55 Toyota Sprint Cup car.
JGR had released a statement Thursday saying Martin was "to drive the No. 11 FedEx Toyota until Hamlin returns."
"Obviously, we're not exactly sure how long that is going to take, but Mark's career speaks for itself and our team knows it will have the opportunity to compete for a win every week," Gibbs said in the release.
Soon after the release went out, sources said, JGR officials were contacted by MWR officials to say Martin would be in the No. 11 at Martinsville and then return to the No. 55 at Texas to resume his part-time MWR schedule. Martin was not scheduled to drive the No. 55 at Martinsville.
Michael Waltrip told ESPN.com that his team was trying to help its Toyota partner by offering Martin for Martinsville.
"We said that we wanted to help, and by helping we would offer up Mark for Martinsville because Brian was racing there [in the No. 55]," Waltrip said. "I guess we just never had the whole deal done when the [JGR] press release went out.
"All we were doing was offering help. We agreed that Mark was driving at Martinsville, and we were working through what it looked like after that. Before we got it all figured out, a press release was put out. I guess it said Mark would drive it until Denny was better, and that was just not what we agreed to yet."
MWR put out a release late Friday afternoon stating its plans.
"Mark is a big part of our organization and is committed to our sponsors, especially Aaron's and Toyota," Waltrip said in the release. "We have a lot of goals yet to reach this year and we are very focused on accomplishing them."
Multiple sources said Aaron's, the primary sponsor on the No. 55, preferred Martin in the car for his scheduled races because the company has activated sponsorship around the 54-year-old driver.
Vickers has recorded two top-10 finishes through the first five races of the Nationwide season. He has 264 career Sprint Cup starts over his 11-year career, posting two wins, 24 top-5 finishes and 63 top-10s.