Like many other senior Indian players, Harbhajan Singh’s decline began a few months after winning the 2011 World Cup. On India’s Tour Of England, a tumultuous tour that served as the harshest reality check for Indian cricket, Singh picked two wickets in around 70 overs he bowled. Many knew he was already ineffective abroad, especially in tests, and his inability was amplified when batsmen didn’t need to actually attack him. 

Bhajji is a very good limited overs bowler. As demonstrated in T20 cricket, he is very effective when batsmen try to dominate him. He gets hammered at times, but almost always gets his revenge. He knows how to fox desperate batsmen that have no choice but to turn it on. To his credit, many spinners have fallen prey to the pace of the shorter formats. 
And despite his prolific test record, I have always maintained that Harbhajan Singh is, at best, an average test bowler. The way he struggled at home in what were to be his last two tests ever in 2013 against Australia—a series that helped him get off 99 and reach 101 tests—was indicative of how his skills and spirit had deteriorated with time. With most of his senior teammates falling around him, Singh retreated into the ranks of domestic cricket, where after a good 2013-14 season, he only managed 6 wickets in 3 matches last season. Those were the only first class matches he played before he found himself miraculously selected for the one-off test in Bangladesh. 
The selectors have made a mistake. Instead of turning to younger spinners like Axar Patel and Chahal, they have gone back to experience—only for a single inconsequential test that will probably have no bearing on India’s immediate future. It’s always nice to see a full-strength side, but surely, Singh couldn’t have made the cut based only on his T20 exploits. With Mumbai Indians winning the title, and Singh consistently picking wickets for his team throughout, he seems to have entered the collective consciousness of more than one national selector. 
This decision is understandable if and only if this is to be Harbhajan Singh’s farewell test match. It has been two years since he played his 101st test, and this should be his last, irrespective of how he performs. His age (35) is not of consequence, but his struggles in the longer formats are there for everyone to see. 
When a batsman has time on his side, and has the luxury of being able to settle down and play at his own pace, Singh is about as effective as Ishant Sharma in ODI cricket. He becomes a containing bowler, much like Jadeja became over the last year or so. That 2013 Australia series seems to have done no favours for India’s test squad composition. Apparently, because of Bangladesh’s army of left-handed batsmen, Singh was given preference over Jadeja. While Jadeja’s exclusion is overdue and justified, this is no real reason to pick an off-spinner to join Ashwin—who already struggled away from Indian turf himself. Karn Sharma must prove that he is effective in the subcontinent, and there are high chances he could be ignored in favour of the two off-spinners, unless India go in with five bowlers for the test. 
Singh’s fighting spirit and aggression is always a boon to have in whatever team he represents, but he isn’t as skillful and consistent as, say, Zaheer Khan—who, despite his age and fitness, will always remain amongst the top 5 Indian fast bowlers at any point. His ability to swing the ball both ways with minimum fuss makes up for his lack of pace. The only issue is his tendency to break down in between test matches and tournaments, which is why younger and lesser bowlers like Shami and Yadav are being persisted with. Give me a fit Zaheer, and even at 40, he will do wonders with the old ball. Bhuvaneshwar Kumar needs some swinging company (pun intended), and in an ideal world, the master would be a perfect mentor for the pretender. 
One doesn’t see that kind of mentorship in Singh, who always looks worried and frustrated when things aren’t going his way. His drooping body language is a reason wickets don’t come his way, even through luck, when he lurks around the field in whites. He begins to blame the fielders and shoot passive aggressive hopeless stares at nobody in particular, as if the world is against him. He forgets that guys like Ashwin and Karn could profit from his advice, even if he is having a bad day. With Ashwin in the same team, India will probably be playing with two fielders less at any point in time. Kohli and co. could let that frustrate them, unless Singh is going to be used as a floating all-rounder in the batting order. His brainless shot-making is an advantage to have, especially because of his ability to shock bowlers with his unorthodox bat-slapping. 
Whatever the case, there is no way this Indian side will be any richer in ability with Singh in the side. Subtract him, and perhaps a youngster will grab an opportunity, or even take an entire tour to grab an opportunity. That’s what rebuilding is about.