Arsenal manager Arsene Wenger says he opted not to risk Granit Xhaka in midfield because of fear of injury.
Welshman Aaron Ramsey started the game but was forced to abandon it because of a hamstring injury against Liverpool with the 25-year-old expected to spend at least two weeks on the sidelines.
The Switzerland international, on the other hand, came on in the second half as he took over from Mohamed Elneny in midfield.
"It could have been [so much better] because I feel we had a great first half," Wenger told Arsenal Player.
"[We were] very dynamic going forward. We dropped in the second half a little bit, I believe a little bit for psychological reasons, just [because of] the 1-1 just before half-time.
"After that we dropped physically as well and 2-1 straight after half-time was a huge blow. I believe that we kept going and we have shown mental strength and overall there are many positive aspects in our game, but we are very disappointed because we lost the game.
"[Granit Xhaka?] Look, him and Ramsey [and others] - they all came back later.
"I pushed Ramsey in and he got injured. It’s a very difficult situation to sort out because the players come back late because of the Euros and, on top of that, you play them and they get injured. [But] if you don’t play them you miss them."