Celtic's transfer window has burst into life, with Mika Baur set to become the latest piece of Martin O'Neill's rebuild.

Reliable reports claim the Hoops have agreed a deal worth up to £6.8 million with Paderborn for the 22-year-old midfielder.

Baur was left out of training while talks advanced and is expected to sign a contract running until 2031 once the formalities are done.

Personal terms were agreed earlier in the window, but Paderborn stood firm after rejecting Celtic's opening bid to secure the Germany Under-21 international.

The Scottish champions have now moved closer to the German club's valuation for a player who helped them win promotion to the Bundesliga last season, and a deal is believed to be agreed.

Baur would bring something different to O'Neill's midfield. He is comfortable operating centrally or further forward, can use either foot and presses aggressively without the ball.

His arrival would continue a rush of recent activity at Parkhead.

Camilo Duran became the first new face when he completed a move worth around £6 million from Qarabag. The Colombian forward was soon followed by club-record signing Kasper Høgh, who cost £11 million plus add-ons from Bodo/Glimt.

Høgh has already made an impact, scoring a first-half hat-trick in Sunday's 5-1 victory over Kilmarnock.

Celtic also reached an agreement for Real Oviedo winger Haissem Hassan, with the Egyptian completing his medical this week ahead of a deal featuring an initial payment of around £6 million and a further £3 million in bonuses.

The attacking rebuild is well under way, but the squad could still change significantly at the back before deadline day.

Celtic 'close' to signing left-back on loan amid Mika Baur deal

According to Stephen McGowan in The Celtic Way, Celtic are close to securing an unnamed left-back on loan.

That position has become a clear priority. Marcelo Saracchi provided cover for Kieran Tierney last season, but his move to Houston Dynamo has removed any possibility of another campaign in Glasgow.

Tierney remains the established starter, although O'Neill needs another natural option to protect the squad across domestic football and a possible Champions League campaign.

A new goalkeeper is also wanted following Kasper Schmeichel's retirement because of a persistent shoulder problem.

Viljami Sinisalo has begun the season as number one, with Ross Doohan offering cover. Celtic have been linked with Nick Pope, although more recent reporting has played down that possibility.

Whoever arrives, the intention is clear. Celtic want greater security behind Sinisalo rather than entering a demanding season with limited senior depth.

Baur may be the next announcement, but he will not be the final piece of O'Neill's summer overhaul.