Wellens completed his move to newly-promoted Leicester earlier this week and although the 29-year-old was happy to remain at the Keepmoat Stadium, the Foxes offered him a four-year deal on wages he could not refuse.
Ryan told the Doncaster Free Press: "We did not want Richie to go. I would have preferred to have kept last season's squad intact and maybe added a couple of players.
"But when clubs come along and offer far more money than we can afford, the plain fact is we are out of the game.
"Richie was happy here, but he was made an offer he couldn't refuse and you can't really blame him for cashing in on what could be his last big pay day."
Ryan added: "We have to face facts because we are a small fish in this league, simply because most of the other teams have around double our average gates."