Nielsen joins the Fir Park side on loan for six months to compete for a place with Graeme Smith.

O'Brien, 20, could start in Saturday's Scottish Premier League opener against Hearts at Tynecastle and admitted it was a relief to complete his move to Motherwell, where he had been on trial since last month.

"I am just delighted to be here," he told the club's official website after clinching a two-year contract.

"The important thing for me just now is to play regular first-team football."

O'Brien had loan spells with Dunfermline and Dundee United in the past two seasons, having failed to make a significant impact at Celtic who accepted an undisclosed fee for his services.

He will have European football to look forward to, after Mark McGhee's side 'Well secured a surprise UEFA Cup place by finishing third in the SPL last season.

"Obviously working under a manager like Mark McGhee is a big pull and all the guys have made me feel really welcome," said O'Brien.

"With the season starting tomorrow and the squad not flush with depth, it gives me a great opportunity to stake my claim for a regular first-team slot.

"First-team football is what it is all about for me now. I've signed for two years and I want to establish myself in the team and make a name for myself and repay the club for having faith in me."

He added: "The pull of UEFA Cup football has been a big one and that is another level I am keen to experience."

Nielsen, 21, joined Blackburn last summer and new Rovers boss Paul Ince will hope the Faroe Islands Under-21 international gains worthwhile experience from his spell in Scotland.

He said "I've come here for six months and I will do my best to push Graeme for the first-team shirt.

"Hopefully I will get into the team and my aim is to improve myself as a player and help the club as much as I can."