Welbeck underwent two operations after suffering a broken ankle in a recent Europa League clash with Sporting Lisbon.
The 27-year-old had featured prominently in manager Unai Emery's cup side and was often used as a substitute in the Premier League.
Emery said in the aftermath of the injury that he would give young forward Eddie Nketiah a chance to fill the void left by Welbeck.
While Sanllehi agreed with the Gunners head coach, he did not rule out the idea that Arsenal may also venture into the transfer market.
"That is absolutely something unplanned," he said of Welbeck's injury. "Yes, that makes you consider things. I am not saying that's a green light to go to the market but we have good players in the house also that may get further opportunities like Eddie Nketiah for example.
"We need to see. We are in November so it's a good time to start analysing what options we have in the market. We are not closing the door but it's still not the preferred one.
"The plans were made in the summer. We are happy with how things are going. We are always going to strive to be the best team we can be.
"If there is a very good opportunity, we are definitely going to consider (it), we will see. Danny Welbeck's injury alters, in a way, the views."