Another hard blow for Lewis' championship chase.
"In P3 (final practice) we noticed abnormal gearbox oil pressure on Lewis's car which we believed we had corrected ahead of this afternoon's qualifying session," said a McLaren team statement.
"This afternoon, as we fired up Lewis's car several times in preparation for qualifying, we became aware that the symptoms were worse than we'd originally diagnosed.
"A decision was later taken to change the gearbox ahead of tomorrow's qualifying session and the grand prix. As a result, Lewis will receive a five-place grid penalty for tomorrow's race."