The three-time Grand Slam runner-up has not played since his shock second-round defeat at the French Open and he admitted after his exit that he had been struggling with a knee injury for several weeks, but the ATP Tour's "rate race" forced him to compete despite being in pain.
Ruud was expected to make his comeback and 2025 grass-court debut at next week's Mallorca Championships, but he has now pulled out of the event, raising concerns that he might not be fit for Wimbledon.
Wimbledon gets underway on June 30 and Ruud is projected to be seeded 15th at the grass-court Grand Slam, but he is yet to make it past the second round at the All England Club and last year he was upset by veteran Fabio Fognini in four sets.
"Just a quick update from my side. I am back on court training. The knee is feeling quite a bit better but unfortunately not 100 per cent ready to compete yet," he told the official tournament website.
"I am entered for the Mallorca Championships next week, which I unfortunately will have to withdraw from.
"I will keep training and pushing to be ready for Wimbledon but Mallorca will not be within reach for me, so I am really sorry for all the people there. I love Mallorca and I love going there.
"I need to listen to the body and I will keep training and hopefully see you back on court soon."
With not too many points to defend at Wimbledon, Ruud could opt to skip the tournament and give himself enough to time to recover fully ahead of the North American clay-court swing.