Henson, 27, injured his right ankle during Ospreys’ EDF Energy Cup semi-final defeat to Gloucester at the Ricoh Arena on Saturday.

Lions head coach Ian McGeechan names his touring party on April 21 and Ospreys physiotherapist Chris Towers confirmed that Henson is unlikely to play again until at least early May.

“Following the MRI scan and specialist consultation on Monday, we can confirm that Gavin has suffered damage to the ligaments on both the lateral and medial sides of his right ankle,” said Towers.

“Although the injury is not so severe that he will require surgery, he will require a spell of at least two weeks in a walking cast before he can start to gradually get back to normal activity.

“At this stage, in light of the nature and extent of the ligament damage, we are predicting a four to six week spell on the sidelines before Gavin will be ready for action again.”

Henson, who made one Test appearance for the Lions in New Zealand four years ago, has endured an injury-ravaged season, starting just two of Wales’ nine internationals.

Henson will miss the Ospreys’ Heineken Cup quarter-final against defending champions Munster on April 12.

With McGeechan taking a trimmed-down squad of around 35 to South Africa, he is unlikely to gamble on any player’s fitness.

In the best-case scenario, even if Henson made it back to first-team action in four weeks he would have a maximum of three Magners League games before the Lions fly out.

That total would rise to five if the Ospreys reach the Heineken Cup final, although the semis are scheduled for the first weekend of May.

England fly-half Jonny Wilkinson, close to returning from a dislocated knee which has kept him sidelined for six months, is also in a race to prove his fitness before McGeechan names the Lions squad.