After playing for the province's second tier side, the Connacht Eagles, Masterson made his debut for the senior team on 23 November 2013. Just months after joining and while still part of the academy, he started in his preferred position at number 8 against Scarlets in the 2013–14 Pro12. He didn't feature again until 23 February 2014, when he started against Zebre in a 19–27 win, playing as an openside flanker following to injuries to senior players Jake Heenan and Willie Faloon. Masterson then started the following four games for the team, during which Connacht recorded a four-game winning streak, their best in 11 years. He finished the season with a total of six senior appearances, all of them coming as starts.