It's unknown when Lowrie, who had been suffering from fatigue symptoms and was put through a battery of tests earlier this week, will return to action. He was not in the starting lineup for Wednesday's game against the Mets at City of Palms Park.
"He has mono," Francona said. "Now, when I say that, I knew everybody's heads were going to [snap up]. He can play as tolerated. There's a reason he was feeling run down last week. He was on the [exercise] bike yesterday. I don't think he's going to feel like he has a ton of energy for the next whatever, week or two. But he can do physical things as tolerated. So we'll monitor him. I don't think we want him kissing anybody, but we'll keep an eye on him and just kind of monitor him. That's about the best we can do."
Francona had mono when he was in high school.
"Everybody's different," he said. "Everybody handles things differently, too. You have no ability to look inside someone's body and tell them how they're feeling. So you just do the best you can."
Lowrie, who was limited to 32 games with the Sox last season due to left wrist surgery and batted .147 with two home runs and 11 RBIs, has been trying to make his way back from the lingering effects of the broken wrist he suffered in May 2008 while with Triple-A Pawtucket. He was Boston's Opening Day shortstop last season.
Pitcher Josh Beckett, who had been out of action with a bad cold, is feeling much better and is expected to make his start Friday against the Pirates in Bradenton, Fla., Francona said.
"Beckett's doing a lot better," Francona said. "That's good."
Left fielder Jacoby Ellsbury is not feeling well and was not in the lineup Wednesday.
"Jacoby's been battling that throat," Francona said. "He's scuffling today. ... We'll let him get looked at by the doctors, get some meds in him and give him the day off [today and tomorrow] -- maybe work him out Friday and get him back in there Saturday [against the Orioles at City of Palms], so we don't prolong that."
Infielder Gil Velazquez, who is batting .294 this spring, was unavailable Wednesday after jamming his thumb Tuesday in a split-squad game against the Astros in Kissimmee, Fla.
"Gil slid into first yesterday and jammed that thumb pretty good," Francona said. "So we're going to get him looked at today. He bounced up, and right away, he had some swelling in there. So ... we're going to have to get him looked at."
Maureen Mullen is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.