The Danish winger limped out of Rangers' 3-2 win against Aberdeen at the weekend and is not feeling optimistic about the injury.
"I shall have an X-ray to check, but at the moment I have major pain in the knee and I am afraid it is serious," Lovenkrands told BT.
"It is terrible timing - ahead of our second leg against FC Copenhagen, which is vital to Rangers.
"It was a special match I would have loved to have played in - at home in Copenhagen.
"I went up for a header and as I landed an Aberdeen player landed on my leg - the knee twisted in the wrong direction.
"I clearly heard a 'knack' in the knee and an unpleasant sensation.
"But I have no experience of knee injuries so I can't say much more."