The 23-year-old Belgian international joined the Gunners in a £10million move from Ajax in the summer and scored in his debut last Saturday in the 6-1 win over Everton.
Vermaelen also impressed in Tuesday night’s 2-0 Champions League play-off win over Celtic.
The Arsenal boss said: “Yes, I am surprised but we saw that he had the physical strength and mentality to cope and when he was needed he came in.”
He added: “I believe he has a good mix of the physical and technical for a centre-back and it is interesting to have a left-footed player because he gives you angles for the pass which are very important for the midfielder.”
