Feyenoord boss Robin van Persie has revealed that Raheem Sterling is feeling "increasingly fresh" as he adapts to life in the Netherlands, and has called on the former Chelsea and Arsenal star to step up with his mentoring responsibilities to the club's young players. Sterling moved to Feyenoord as a free agent in the January transfer window. He has thus far made four appearances for the Dutch side, with his minutes appearing to steadily increase. His first two appearances came as a substitute, in a 2-1 win over Telstar and a 2-0 defeat to FC Twente; he also played 63 minutes in their 3-3 draw with NAC Breda, before registering an assist against Excelsior in their latest 2-1 victory last week. Now, the former Arsenal and Manchester United striker Van Persie has issued an update on the winger's fitness, insisting that he is getting closer to being able to play for 90 minutes every week, while he has also credited his ability to mentor young players. Per Ed.NL, he said: "He's getting fresher in his actions. You saw that on Sunday with his assist and dribbles. He continues that line. He talks more and more to players on the training field about his period in England. You see him growing in that. It's important that he takes that role."Van Persie has also accepted that Sterling was unhappy to be substituted in their clash with NAC, but Van Persie believes it shows the winger's passion for the game. He added: "He was disappointed at the match against NAC that he was substituted after an hour. I think it's nice in a way, that's the enthusiast in him who wants to play. The previous match was 80 minutes. He's not limited in minutes anymore."Sterling himself admitted as much, adding: "I felt good enough to continue, but it is the coach's choice. Every player wants to stay on. You want to stay on the pitch. It is understandable that it is not yet possible. It is another new step in the right direction."He added: "We would have liked to win. We have mixed feelings about the performance. It was very inconsistent, but at times also very good. I want to get back to my old level. It was a nice attack from our side. We have to keep working on it and then more will follow."Sterling has already been criticised during his time in the Netherlands, with Dutch pundits regularly questioning whether or not the winger was capable of playing in the Dutch top-flight after months away from the game after his dismal Chelsea spell. Van Persie, though, has leapt to the winger's defence, saying: "I thought it was misplaced. If you look at the context, where he's coming from and what he's already accomplished, he hasn't had any team training for months. I understand there are expectations. What he's done is extraordinary. He's an absolute top player who's won almost everything. To then criticize him like that based on half an hour... I think that's completely misplaced. You really don't understand it at all."He has also pleaded for time from critics, adding: "It's always allowed (to analyse performances), but I would say it's better when we are six to eight weeks further along. If you then find that he isn't delivering and you can back up that opinion, then I say: okay you have a point. But throwing out such strong opinions on the basis of half an hour, I think that's inappropriate."Add GOAL.com as a preferred source on Google to see more of our reportingFeyenoord play Ajax in a huge game this weekend. Van Persie's side are second in the Eredivisie, although they sit 16 points behind leaders PSV Eindhoven. Ajax, meanwhile, are fourth, five points behind Feyenoord; this is a major game in the race for Champions League qualification.