Owen Hargreaves may be forced to quit top-level football in the next two years if his chronic knee problem persists, BBC Sport understands.The Manchester United and England midfielder has had two operations to cure a tendinitis problem in both knees but the pain has yet to subside. [LNB]It is believed the 28-year-old has not yet planned a return date but is still confident of making a full recovery. [LNB]He has had only 25 starts since a £17m move from Bayern Munich in July 2007. [LNB]Hargreaves, who was seen demonstrating his ball skills during United's warm-up for the Champions League final in Rome on 27 May, has not featured for Sir Alex Ferguson's side since their 1-1 draw with Chelsea on 21 September 2008. [LNB]He underwent surgery on his right knee last November and then on his left knee in January. [LNB]The procedures were carried out under the guidance of leading American knee surgeon Richard Steadman, who has previously operated on former Newcastle striker Alan Shearer, current Magpies forward Michael Owen and ex-Red Devils striker Ruud van Nistelrooy, who is now at Real Madrid. [LNB]Ferguson has previously said that he was "certain" Hargreaves would be fit for the beginning of the 2009-2010 campaign but it is now believed there is little chance of that happening.[LNB] Hargreaves is different to any other player in the world - his experience at the top level and desire to win makes him an untouchable asset to the Man Utd and England squads redevils99 Hargreaves, who is halfway through a four-year contract at United, hopes to begin training with the United first team at the end of July or the start of August but he could be restricted to as little as one session per week. [LNB]Manchester United declined to comment on Hargreaves' current level of fitness, but it is believed that his injury is so delicate it can only be monitored on a day-to-day basis and stepping up the intensity of his rehabilitation programme could have potentially disastrous ramifications. [LNB]If the England international suffers a recurrence of the original problem he would face another lengthy absence. [LNB]Hargreaves aims to manage the injury as slowly as necessary before making a return when he is pain free but he will not return if he is feeling any level of discomfort. [LNB]If Hargreaves is pain free he is likely to be able to play at the top level for at least four or five more years but, if not, a premature end is likely. [LNB]His condition, patellar tendinitis, can be described as inflammation of the tendon that attaches the quadriceps (thigh muscle) to the tibia (shin bone). [LNB]It can be caused by a number of factors, including a rapid increase in the frequency of training, sudden increase in the intensity of training, transition from one training method to another, genetic abnormalities of the knee joint and poor base strength of the quadriceps.