That's when the veteran lefty was struck in the head by a line drive off the bat of Desmond Jennings. The scary incident caused a minor fracture in Happ's skull, but it's the knee that he twisted on his way to the ground that has proven to be the biggest obstacle to overcome.
"His head is feeling fine, his knee is bothering him a little bit," manager John Gibbons said. "When he fell after getting hit he twisted his knee, so that's the big concern right now.
"Somebody's looking at it all the time but no more X-rays. It's just one of those things that's just going to take time. He'll be a while."
Happ's timetable for a return is still somewhat unclear. General manager Alex Anthopoulos was recently quoted as saying he expected Happ to be out anywhere from three to six weeks.
The three weeks would seem highly unlikely as the native of Illinois has yet to throw off a mound, and six weeks would have Happ back in action toward the end of June.
Happ is 2-2 with a 4.91 ERA while striking out 26 in seven starts this season.
Gregor Chisholm is a reporter for MLB.com. Read his blog, North of the Border, and follow him on Twitter @gregorMLB. This story was not subject to the approval of Major League Baseball or its clubs.