The Norway international is on loan from Real Madrid for the rest of the season.
He was in fine form as he scored the opener in a 3-1 away win for Arsenal against Olympiacos in the Europa League round of 16 first leg.
"He helps us to do the processes much better," our manager told reporters. "He helps us as a team because he gives us a lot of continuity on the ball.
"He's really intelligent to find the spaces, he attracts a lot of opponents. We ask him to score goals, to arrive in the box.
"Today, he scored a brilliant goal and his contribution was really important for the team."
He added: "I think he was really good again tonight.
"I wanted to give him some continuity because he deserves it. He's training really well.
"Another assist and then Elneny, he's been practising those shots from outside the box with Roundy and today he got his rewards. I'm really happy with the subs as well, the way they came on."
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