England winger Jonny May suffered a suspected dislocated elbow as Adam Hastings' amazing drop-goal helped Gloucester beat London Irish.
May fell awkwardly and went off with his arm in a brace, while England team-mate Henry Arundell also limped off.
The hosts had led 18-12 at half-time after tries from Agustin Creevy and Ben Loader, while Santiago Socino and Ruan Ackermann crossed for the visitors.
Socino added a second try before Hastings' winner from his own half.
Exiles teenager Arundell was the first to depart after colliding with his own team-mate Loader and suffering an ankle injury.
May also tangled with Loader as they competed in the air for the ball and landed heavily.
After several minutes of treatment on the pitch, the 32-year-old left the field with his arm protected and is now a major doubt for the upcoming autumn internationals.
May and 19-year-old Arundell were both named in Eddie Jones' England squad for the series, which starts with a game against Argentina on 6 November.
May's injury took some of the gloss off a victory that moved Gloucester up to third in the table, while Irish could finish the weekend bottom of the league if Bath beat Northampton on Saturday.
The home side made the perfect start as Creevy gave them the lead inside three minutes, as they took advantage of a penalty to kick to the corner and drive over the line.
Gloucester overloaded the right hand side for Socino to get over the line in the corner and Loader outjumped May to catch a pinpoint crossfield kick from Jackson and score.
Tries from Ackermann and a second from Socino brought Gloucester within three points before they finally took the lead in remarkable fashion.
With seemingly no threat and the ball in the middle of the park, Hastings unleashed a huge kick from his own half which swerved and dipped just over.
The hosts had substitute Ciaran Parker sin-binned for trapping the ball with his feet, but neither side could add to the score as the visitors held on for the win.
London Irish director of rugby Declan Kidney told BBC Radio London:
"You look how little the drop-goal went over the crossbar and that's probably a good indicator of how these matches are decided.
"What we have to do is take it on the chin, suck up the disappointment for today and tomorrow, roll our sleeves up and get back to work on Monday morning."
On Arundell's injury: "He's had a bit of a tweak on the outside of his ankle so that'll obviously take a little bit of time to settle down.
"When people see crutches they can catastrophise it a little bit but it's a twist of the ankle so the medics will talk with the England medics and then I'm sure Eddie [Jones] will decide whether to bring him to training camp or leave him with us for rehab."
Gloucester head coach George Skivington told BBC Radio Gloucestershire:
"Jonny obviously went off with his arm early on. Away from that I don't think we were too bad.
"I know Arundell went off on the other side, that's two of the England back three potentially not in a good way.
"It's part and parcel of the game unfortunately and I'll find out tomorrow how bad everything is."
On Adam Hastings' drop-goal: "It was brilliant. These guys have licence to use their skills when they see opportunities.
"That was a seriously good drop kick from Adam."
London Irish: Arundell, Loader, Luna, van Rensburg, Hassell-Collins, Jackson, White, Fischetti, Creevy, Chawatama, Ratuniyarawa, Simmons, Gonzalez, Pearson, Fa'aso'o.
Replacements: Miller, Goodrick-Clarke, Parker, Munga, Cunningham-South, Englefield, Joseph, Parton.
Gloucester: Carreras, Rees-Zammit, Harris, Kveseladze, May, Hastings, Chapman, Ruskin, Socino, Balmain, Clarke, Alemanno, Reid, Ludlow, Ackermann.
Replacements: Singleton, Elrington, Knight, Jordan, Tuisue, Varney, Reeves, Evans.
Referee: Matthew Carley