He played eleven matches and scored one goal against Hearts,[2] becoming popular with United fans who voted him their young player of the year.

[5] Eustace started the 2002–03 season well and attracted interest from Premier League club Middlesbrough, to whom he was loaned out for one month in January 2003.

At the start of the 2003–04 season, Eustace left Coventry for fellow second-tier team Stoke City on a free transfer.

[8] He made his debut in a 3–0 win over Derby County at the start of the 2003–04 season and helped the side to 11th place in the Championship, scoring five goals.

[11] He made his first appearance for Stoke City in over 18 months in their 2–0 friendly win over local opposition Newcastle Town.

[16] He was a regular for Stoke during the first half of the 2007–08 season and captained the team up until his move to Watford on the last day of the transfer window.

[18] On 9 March 2009, he joined Championship rivals Derby County on loan until the end of the 2008–09 season, in order to help solve the Rams' midfield injury crisis.

[20] Eustace was unable to secure a permanent move to Derby at the end of his loan spell with the club and returned to Watford at the beginning of May.

"[22] With his Hornets career seemingly in jeopardy, Eustace signed an amended contract stipulating that he would not receive his sizeable appearance fee, freeing up Malky Mackay to pick the tenacious midfielder.

[27] Eustace kept the captain's armband following the managerial appointment of Sean Dyche in June 2011 and signed a new two-year contract until 2013 at the end of July despite interest from Derby County.

[28] In June 2013, Eustace was offered a new contract with a player/coach role, which he declined, stating that he was not ready to turn away from playing just yet, therefore leaving the club as a free agent.

[29] In July 2013, it was confirmed by Derby County that Eustace had rejoined the club on trial, with a view to earning a contract[30] and after impressing Nigel Clough in pre-season training and friendly games, Eustace verbally agreed a one-year contract with Derby[31] and the move was made official on 24 July.

A focal point in defensive central midfield, Eustace had only been on the losing side once in 14 appearances prior to his injury for Derby during the 2014–15 season.

[43] At QPR, he had a brief spell as caretaker manager in 2019,[44] helping the club avoid relegation from the Championship with 7 points from his 7 games in charge.

[39] Following Mark Warburton's arrival as manager in May 2019, Eustace remained as his assistant and began studying for his UEFA Pro Licence.

[39][45] In March 2022, he joined the Republic of Ireland national team in the same capacity, working under Stephen Kenny, combining this with his existing role at QPR.

Eustace gained his first victory as Birmingham manager in the following match day, beating Huddersfield Town at home on 5 August 2022.

[55] When Eustace joined, Derby were only out of the relegation zone in the Championship on goal difference, with Eustace admitting in his press conference the risk of leaving a battle for a play-off place at Blackburn to join a relegation battle at his new club, but he felt the Derby job was a better long-term prospect.